So, there were different rear differential manufacturers in 1964 F-85 cars, what determined what each car received? I know there were standard gears (and severla options) for model and engine/transmission combos, but what determined the casing?

 

assembly plant location & variety of makes/models built at that plant ??

 

Are you asking about the axle housing? Buick vs Pontiac? The rear end ratio determined what rear you got.

 

The exact same 64 jetstar axle - less differential & complete differential in my 63 98 conv in lansing michigan I informed you of before & 70 miles north at merlin melroses place should be some more....