late 50's gm axle id

What does a large P cast into the third member housing mean? and are the 3rd members interchangeable from chevy to Olds? I also noticed that some years had a deeper bearing cap on the axle ends, what years are what?

 

the large P probabilly stands for Positraction , at least it does on 55-64 chevy pumkins

 

Chevy is by itself. Olds and Pontiac are interchangeable, but '57-'58 center sections take coarse spline axles, and '59-'64 take a finer spline.

 

That's pretty much what I had heard, but what about the deeper bearing cap? The housing is slightly different too for those.

 

is the third member round, or oval? round is olds/pontiac, oval is Chevy.