I have a 1970 Cutlass Supreme Convertible with a 12-bolt rear end. Was that rear end standard or an upgraded option?

 

It's an O-Axle. 12 bolt cover with a 10 bolt ring and bolt in axles. Standard on all Oldsmobile cars from 67-70 except Canadian built cars.

 

Thank you very much for the clarification!

 

Yes, last year of the O axle. In 1971 went with corporate rear end with bolt in axles (I think).

 

And to clarify further, it is not a "12 bolt" axle. The number of bolts refer to the bolts holding the ring gear to the carrier. It is coincidental that on Chevy axles the number of bolts on the rear cover matches the number of bolts on the ring gear. That is not normally the case for most rear axles. The Type O axle uses ten bolts to hold an 8.5" ring gear to the carrier. The Chevy 12 bolt uses 12 bolts to hold an 8.875" ring gear to the carrier. Completely different axles. Nothing interchanges.