GMC offered a Fuller R-950-C, A Spicer 8031, and a Spicer 6231 in those years. Its kind of hard to tell much with all the dirt, but it looks like the Fuller to me. However, it didn't necessarily have to come out of a GMC. In those days, we went to a salvage yard and looked for an auxiliary box and didn't really came what it came out of. In fact salvage yards usually had them laying in a shed with no reference to what they were removed from.
