I'm at work today (but had a USB stick with scanned manuals handy) but had always understood the major difference between the various 60cwt transfer cases was that the 60X had different internals to allow for the torque of the driven axle compared to the winch on the other models that was used less of the time. However - looking at the parts manuals, early 60S, 60L and 60X all had different part numbers for the case and cover assemblies but the last version of all 3 used a common part number. I don't have any idea at this point what the detail differences were but there must have been at least some small detail different to justify new numbers. They seem to have supplied the housing (case and cover assembly) only as assemblies, not the case or cover separately so I can't tell whether the cover might have been common to the C15A. The gasket between case and cover is listed as the same part number for all versions of C15A, C60S, C60L and C60X transfer case so the interface between the case halves must have been the same size, shape, assembly screw locations etc.
The frame cross member assemblies for each model are different with several numbers listed for each truck model. For the 60S and 60L, the part numbers are different for 1st and 2nd type cabs (11/12 and 13). For 15A, the numbers vary depending on whether the frame had reinforcements (later) or not (earlier) and a 3rd, later number for cases with "mounting bushing & cushion", but no mention of the cab types. For the 60X, the difference is tied to the thickness of a spacer (early 43/64", later 33/64"), (I'm not aware of 60X having ever been built in cab 11/12 configuration). At this point I don't know the differences between the crossmembers. Field observations will be needed - later in the week.
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