It is definately an 8.5" 10-bolt, identified by the cast corner "blocks" at 5 o'clock and 7 o'clock just below the cover face. I don't see a posi tag(it would have been an stamped aluminum tag about 1-1/2" long x 5/8" wide saying "use positraction differential lube only", attached to a cover bolt, but they frequently corrode and rot off) so you'll have to remove the cover to know for sure what is inside.
Width measurement needed is wheel mounting surface to wheel mounting surface. Just off hand based off the width you have given and the suspension brackets is that it is 73-77 A-body, but that is just a guess at this point.
A picture of the end of the housing where it meets the brake backing plate would help, as a look at the bolt pattern and size could narrow ID down further. There is the possibility it is a Type O(Oldsmobile) 8.5" and the housing end is different.
Could also be an 8.5" from a full size GM(Caprice/Impala) depending on total width.
The #'s you have posted won't help.