I think you're right.,.. posi units were more common that most realize. (although not super plentiful)

The posi option was not that "rare" out here in the west.  A lot of people today don't realize that the reason for ordering this option was not because they needed to nail all that 6 cylinder or 283 power to the ground.  It was for one thing ,..... more traction for plowing through snow and mud.  And this was back in the day when a lot of roads out here were not maintained that often after rain.  In fact ,. out her in Manitoba,.. there were lots of dirt roads at one time,.. and farmers who lived down these roads.   So a posi was a "needed" option for some buyers.

And yes,.. if you were considering that 425 HP option,.. yes maybe you were also considering a posi.   But I would guess taht almost all of the regular drivers in '64 who "needed" a posi,.. were those who lived on counrty roads,.. lots of farmers,.. and those who considered it a "need" for snow.

There was time I would have punked down a lot of money to find a posi and have it installed.  Brag rights for a posi indicated "high performance" option if you had one. With a mighty 283 ,.. what was I thinking ?  Not like I was mud bogging or driving in snow either. 

The US Pontiacs have a lot more cars that have rare combinations.  They're a whole different beast of course.  But there are lots of cars down there that have a whole range of amazing combinations from 8 bolt rims, lots of interior combinations and many more cars with air conditioning.

Up here ,.. factory tilt wheel was a "luxury" option and not often seen.  The full "stereo system" with reverb switch.  It was also uncommon to see a Stato Chief with an 8 cylinder.  Most people bought those with economy (cheap) in mind.  If there are any Strato Chiefs out there with a lot of extra options ,.. that would be cool.,.(although I've never seen any).  It's wild to think that there was time when white wall tires were considered and "option"!