Those codes should either be stamped on the axle tube or on the front face of the 3rd membber, depending on year. The coes I listed are for the 4.11 gear, there's plenty of other codes, too, for the 4.56 and 5.13 gears. I even came up with one for a 6.17! The info came from the Catalog of Chevy Truck ID Numbers from Cars & Parts.
Those 2 eaton axles were the ones used in the lighter trucks, the HO52 being the 3/4 ton version and the HO72 used in 1 tons. There really isn't much differences in them, other than perhaps brake size. In fact, I wouldn't be suprized at all to find a few HO72s under 3/4 tons, especially if they were equipped with the dual wheel option.
