Swami29 - Do you have pictures of the unit? My early '60s GMC resource info is limited, but in comparing what I have with the Chevy and GMC parts books, the 1960-62 differential carriers should have the casting# "3707340" (GM Part# 3718011) which were used for Chevy and GMC differentials for those years.
This applies to the H033/H035 rear axle assemblies.
Sounds like you found pay dirt if those numbers match.
