Thanks for that detailed explanation Bill. As I mentioned I had limited GMC resources to look at in comparing the Chevy vs GMC rear axles. The screenshots below from a 1961 Chevy Master Parts Book and a 1963 GMC Master Parts Book both showed H-033 differentials listed as GM# 3718011 and GM Casting# 3707340 for 1960-62, which is where I drew from for my conclusion above.
I guess what confused me about the GM Master Parts Book is it listed this H033 differential carrier for applications "X" and "Y", which if IIRC stands for 1956 and 1955 respectfully, but also for "J" , which IIRC stands for 1960-62,for the GMC H035 differential.
See my confusion point?
Anyhow, regardless of this info, a picture of the sellers offered differential should help us resolve Swami29's question on if it will fit his axle housing or not.
