There were some optioned a-body cars that came with a heavy duty 10 bolt cover rear and 12 bolt guts rear. We have one here right now with 3.42 gear and eaton factory posi. Large 11" DRUM BRAKE OPTION and still 5 on 4 3/4 GM bolt pattern for mid size cars.
My '76 Century Special came with a V-6 and 2.73 open rear. It has what can be called the "corporate" 10-bolt rear which is good because you can get plenty of parts for them and they are very tough. The best part is it still has the bolt in axles instead of the c-clip used in the Chevys. My car also came with the 11" front discs and the 11" rear drums which is standard for these cars. They were all upgraded from the previous intermediates(1972 and earlier) because of the weight I believe. I've never heard of 12 bolt carriers inside of 10 bolt cases(stranger things have happened) but Olds did have 12 bolt cover rearends with 10 bolt insides. They're generally not worth their trouble.
THe 1965-1970 Pontiac full size cars had a 10 bolt (8.2 looking ...LOOKING cover) but were really 12 bolt rears and 31 spline axles. This continued on into the 70's rears but some were 28 and some 31 and then switched to c-clip axle retention too. Jim
