['ol Pinion head]

Originally Posted by Jay S

When looking from the back the other 8.5s have the triangle shaped extrusions that stick out on each side on the bottom of the case right in font of the cover.

There are actually two different '71-72 versions of the Round Cover Olds 8.5 A-body housing. the more common A9 housing end style, & the larger A10 axle bearing style with its special backing plates.

What's nice, is the round cover used on c-clip housing 8.5 housings is a direct bolt on to the Olds, so I always swapped on the round cover for outright sales, ESP total builds to present at the big swappers.
Early on in the early 90's, on many an 8.5 A-body build I also carefully ground down the big raised A's & O's off the different center housings. Also ground off many a casting number. The reason was to throw off wannabe future competitors, this was when i was selling fresh built up 3.08- 4.10 ratio 8.5 posi rears. It took longtime early 70's diehard Olds 442 or Buick guys to have a clue of what the "Big10" rears were. At the time, so many of the Olds & Buick guys didn't pay attention, and the Checy guys were clueless. They were also of the drift that their car needed a 12 bolt posi, so they totally ignored all the inherent weaknesses of a stock 12 bolt posi "Chevelle" rear. I used to call it the "I gotta have a 12 bolt" syndrome, LOL.