The only information stamped on the hub is 4130. I'm 100% sure this is the type of steel used for the hub. Being made of 4130 steel would make this hub near indestructible. It had a very old 5-speed Shimano freewheel. I had to take the wheel to a bike shop that’s been around forever to get it taken off. My LBS did not have the tool to remove it and neither did I. The mechanic told me that tool might not be available for sale anyway. It looks like a standard Shimano freewheel tool would fit except the hole in the freewheel is too small for it to fit. I had to remove the axel before the bike shop could get there tool into the freewheel to remove it.