What rearend do I have?

[rs74]

I have an 85 short bed that came out with a 10 bolt new. Some years back I swapped in a 12 bolt. I have no idea what year this rearend is. I am replacing all the brakes front and rear. I was going to replace the rear shoes, wheel cylinders and a new hardware kit. The rear shoes are 2 3/4" wide. The meat on the front shoe is 10" and the meat on the rear shoe is 12". The parts store doesn't even know what I might have. I figure it is a 70's rearend but know idea on what exact year.

There is no tag on the rear cover either. Is there any way for me to figure out what is in this thing so I can get parts.