Which rear? 8.2, 8.5 10 Bolt???

This is the rear under my 1951 Mercury custom. My car is running a newer GM frame, was reportedly a big Pontiac though it has stock SBC motor mounts so I'm not sure. Any idea what this rear is? I have dreams of adding a posi, as I have an open rear end...

 

I think all of the 1973 and newer 10-bolts are 8.5".

 

Looks like a B.O.P. I think they are 8.2 and have bolt in axles (no C clips).

 

The picture is of a 8.5" rearend out of a 70's GM car. In the last 4 years I've installed two 8.5" rearends.

 

Thanks guys.

 

this is an 8.5 the 8.2 dont have the square corners sticking down on the bottom under the cover

 

That's good to hear, easier to find 8.5 parts and gears.