The only way to know for sure is to contact GM Vintage Vehicle Services with your serial number. It will cost you somewhere around $100 but you will find out how many made, all options it had, what dealer sold it etc. A good investment, especially for rare cars.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
If you want to know if it has the original engine check the stamping number on the block. I believe the last numbers should match the last numbers of the VIN. Someone correct me if I'm wrong. As far as the rarity of these cars, there were only 741 made total 2+2 convertibles. So even if it's not an original big block car, it's still quite rare.
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'68 Parisienne 2+2 Convertible Matador Red (Resale Red but not for sale).
