Saginaw tranny's have rings on the input shaft, and cast iron cases. The rings tell you how strengh of the tranny. I would not use them on motors over 400hp or if you into really hitting the gears with a posi rearend.
I have 3 of them in my garage. 2 came from '75 Camaro's, I don't know about the 3rd.

I have pic's of the shafts somewhere but can't find where I put them!

Pat