Did you mean the bolt-on dual hubs that have the 3 u-bolts in them? Those became available sometime during the 50 series. 

If you mean the axle carriers, the cast iron housings that bolt directly to the rear end housing, then there were 7 different parts numbers listed in the parts book. There should a part number cast right into the axle housing, let me know what that is and I can tell you more. It could have the Wheatland axle housings in it for some unknown reason.