I know that they're not Pontiacs but hope that this is permitted/of interest:

THE ROYAL McLAUGHLIN-BUICKS

The first Royal Buicks were various 1924 Models which were used when the then Prince of Wales opened a massive dock in Southampton, and a photo of the Prince getting into Southampton registration CR 9161 appeared in CANADIAN AUTOMOTIVE TRADE magazine of September 1924. The caption reads At the opening of the new dry-dock at Southampton, England, just prior to his trip to Canada, HRH. the Prince of Wales used nine Canadian built McLaughlin-Buick motor cars to transport himself and [his] party.  This is the first time that any but English cars have been used by British royalty in England. The actual car was registered with CR 9161 in Southampton County Borough, but the details of the registration do not exist. It is believed that the car was actually given to him as a present, just before he sailed for Canada and just after he opened the British Empire Exhibition in April 1924 at Wembley, as the Canadian centre had a large General Motors of Canada display in it. These are the details of the two similar cars from registration records:

CR  9162 BUICK MONARCH 7-SEATER TOURING , registered in 1924

CR 9161 BUICK MAJESTIC TOURING GREY 27.3 H.P. 6-CYLINDER 1924 BORE 3 3/8 INCHES SERIAL # 89839 ENGINE # 1171579 registered 10 July 1924

On 27 June 1924 the Prince of Wales rode in CR 9162 when officially opening the Prince of Wales Floating Drydock in Southampton. Nine cars were used for the official party, and this was the first time that the Royal family had used anything other than British cars. The occasion was just before the Prince sailed for Canada, and the U.S.A., and shortly after the opening of the British Empire Exhibition at Wembley 23 April 1924, the Canadian Pavilion being the most visited. A photograph of the car with the Prince getting in was published in Canadian Automotive Trade, September 1924. It is believed that the cars were all supplied by local dealers, Southern Traction Limited of Southampton, and registered, temporarily in some cases, CR 9161 to CR 9168. Southampton City Council records show that CR 9161 was a 6-cylinder 27.3 H.P. Majestic Touring Model 24-X45 painted grey Serial # 89839 Engine # 1171579, on the short chassis. CR 9162 was it seems a 7-seater Monarch Touring Car, Model 24-X49, but as the car appears to have been sold or transferred to another County, there are no records of the issue of the number and it was not re-allocated. CR 9163 was a 4-cylinder 18.9 H.P. Majestic Touring in grey, # 81606 Engine # 1100295. CR 9164 was a four-cylinder 18.3 H.P. Model 24-X37 Saloon, Serial # 82245 Engine # 1105067. CR 9166 was a Model 24-X49 Monarch 7-seater Touring Car in yellow and blue, Serial # 91431 Engine # 0013287. Finally, CR 9168 was a 4-cylinder 18.3 H.P. Coupé in Maroon, Model 24-X33, Serial # 80773 Engine # 1110096. The missing numbers must have been for Buick cars that were sold and the numbers re-used, although in two cases we know that the numbers were allocated to 1925 Model Buicks sold by Southern Traction.

At the Exhibition the marques exhibited included various GM of Canada-sourced cars: a Superior Chevrolet Tourer, and at least one Buick Tourer [Models 24-34 Standard Four or 24-55 Master Six], an Oakland Tourer and an Oldsmobile Tourer. The Oakland was in fact a Model 6-54A, one of 783 Oaklands built in Oshawa, and the Olds was a Model 30B six-cylinder Tourer, one of 1,481 Oldsmobiles built in Oshawa in 1924. Three photographs and a brief reference appeared in General Motors World in July 1924.

Then, as illustrated on the front cover of Driving Force by Helen Earley [car in National Museum of Science & Technology, Ottawa, as is the 1939 Royal Touring Car] For a tour of Canada in 1927, General Motors of Canada Limited provided two special 1928 Model 28-496 Touring Cars, one of which was Serial # 139645, Engine # 2034106, painted in Desert Sand with turquoise stripe, and seated seven with lizardskin seats and spare wheel cover.

1928 MODEL McLAUGHLIN-BUICKS [2 OF] MODEL 28-496 TOURING CAR

NMST cars is Serial # 139645 ENGINE # 2034106. This would be a Model 28-49 Touring Car in U.S. parlance.

The first Royal McLaughlin-Buick in the next decade was acquired by a member of the Royal Family, Lady Patricia Ramsey, who was a granddaughter of Queen Victoria. She ordered a Freestone & Webb limousine to be built on a 1934 NA chassis. The chauffeur, Fred Rix went to the Willesden works [New car Production Department] daily to see the building of the chassis, and then to the coachworks. The car was registered AYL 237 in London in 1934, and was built especially tall so that the owner, who was over 6 ft. tall, could enter and alight gracefully. The car was used well into the Sixties.

The most famous Royal [McLaughlin-] Buicks were the 1936 models of H.M. King Edward V111, though there were in fact several cars acquired between 1936 and 1939 by both the King, later H.R.H. The Duke of Windsor, and his brother, the Duke of Kent: both series 49 Limited and 48 Roadmaster.

Mr Johns recants the tale that during the Autumn of 1935, the then Prince of Wales paid a visit to the Albemarle Street showrooms of Lendrum & Hartman, and Captain Hartman himself was called from a local hairdressers to personally attend on his Royal client. The works manager, Ted Taylor, who had been recruited by Hartman from the Buick Company, then had an appointment to visit the Prince at York House to take instructions on the requirements of the client. Taylor was then sent to Oshawa to supervise the building of the two cars ordered: one the main car, the other a back-up. The cars were in fact Model 4933 Limited chassis, with special Limousine bodywork. The fittings included a 2-wave radio with controls in the rear-seat armrest, the rear quarter windows were replaced by panels, and inside mirrors with recessed lights, and folding rear lamps were covered by sliding panels. The rear window was only 5 in. high, which ultimately necessitated a rear-view mirror mounted on the side-mount. The rear window could be covered by an electrically-operated rear blind. Two Batteries were fitted below the drivers seat, with a double-pole throw switch enabling the spare battery to be switched in an emergency. Marchal headlights and fog lamps were fitted, with wiring modified to reduce voltage drop. A Smiths Jackall hydraulic system was fitted, together with Goodyear Lifeguard innertubes to make blowouts unlikely. The interior fitments of the special car included a pipe-rack, ****tail-shaker, drinks and glasses cabinet, a food container, cutlery, telephone directories. Gold cigarette boxes presented by Captain Hartman, and a holder for Swan Vestas smokers matches. There was also a second car, which seems to be forgotten about, which was back-up car. The first car was unusually registered, as Royal cars in the ownership of the King, as the Prince had become in January 1936, did not carry registrations though other Royal cars in the Royal Mews were registered, though often with special numbers. The first car was registered CUL 421, which was in March 1936, the registration following shortly after a large batch of London Passenger Transport Board buses! This car was driven generally by Mr George Ladbroke, Head Chauffeur. There was clearly no effort to allocate a special number and perhaps this was for reasons of anonymity? The second car was given the registration CLN 6 and was clearly delivered slightly later, registered in London as before in June 1936? The other car ordered was in fact purchased by Mrs Wallis Simpsons second husband, Mr Ernest Aldrich Simpson an American-born British naturalized businessman living in London, for her personal use, and was delivered at the same time as CUL 421, but allocated CUL 457, and driven by a Mr Wagstaff, Second Royal Chauffeur. This car was not a Model 4999, but a series 48, Model 4899 Roadmaster chassis four-light Limousine built on the series 48 [U.S. 80 series equivalent] chassis by Oshawa. Both cars were ceremoniously photographed together outside the Buick House emporium with Captain Lendrum standing behind. A photograph of either of the two original cars was featured in the London Motor Show advertisement for Buick in 1937 Show Catalogue. CUL 421 was registered 6 March 1936 and carried Serial number 649990164 and Engine number 2943885

I have just been watching a BBC documentary on Edward and Mrs Simpson and just after he abdicated it shows the king leaving  Fort Belverdere in CUL421 followed by a Buick limo with the rear quarter windows intact and a large rear window, the reg no was  CXF 436 as far as I could make out..

Later in the season, the Duke of Kent acquired a 1937 Model 49 Limited Limousine, a standard model, which was registered with his personal registration YR 11 [London County]. This car was then exchanged for a special 1938 Model with wind-down black glass to the rear quarter-lights. This new car had the registration transferred to it as well, so there were two YR 11 cars. This was actually ordered by Princess Marina, Duchess of Kent, and was fitted with purdah glass that could be wound up to obscure both rear quarter-windows. Servicing was carried out at her Belgrave Square address, and the car was driven by chauffeur Mr Field. A photograph of the original YR 11 was published in General Motors Worldof December 1936, which described the car as A special Canadian-built, McLaughlin-Buick 90 Limousine, purchased by His Royal Highness the Duke of Kent, through Messrs Lendrum and Hartman of London-Since the car is to be used for official purposes, it is fitted with the Royal Crown and with the Royal Ensign. The car naturally had dual sidemounts, plus fog-lamps.

There were to be two further special Model 49 Limited models ordered by the Duke of Windsor in 1938 and 1939 whilst he was living in France, though R.H.D. and four-light configuration as per CUL 421.

It is believed that the Royal cars were actually built, as were the two special 4900 series cars used by King George V1 in Canada in 1939*, by the Oshawa Passenger Body Tooling and Production Engineering Department. In 1939, the Department was under Mr F.E. Hudson, and Mr A. Maynard was Chief Engineer of G.M. of Canada. However, Lendrum & Hartman were directly involved as has been shown, and illustrates the close connections between Lendrums and G.M. of Canada, which were completely outside the scope of General Motors Limited in London. However, history has shown that the public thought of the cars as American and associated their American overtones with that of Mrs Simpson who had caused their King to abdicate. It is doubtful, with respect, that the cars could have been more Canadian, or at leats Anglo-Canadian, and these sentiments are to be regretted.

*One of the cars is currently held by the Canadian Museum of Science and Technology, Ottawa, and both cars were Model 4929 (Series 90) Royal Touring Cars, the car in the Museum being serial # 9492902502, Engine # W3422857, i.e. a Walkerville-built unit. The chassis was an extended 155 wheelbase 4900 series, and the body a four-Door 7-seater Touring Car, 29.