8 3/4 rear axle id......
SBP axles would be the first clue if it was out of an A-body. 741 case would be std fare for an A-body 8.75
Look at where the housing vent/brake tee attaches.
Other than the painted on markings, don't know of any specific stampings like on a Dana 60 for 8.75 rears.
Post a pic if possible.