The original use of the term referred to the maximum recommended payload. For military trucks made in the 40s and 50s, the nominal designation referred to the "off-road" payload. Hence, a 2 1/2 ton truck had a payload rating of 5000 pounds off-road, or 10,000 pounds onroad.
For your '51, see the complete specs with GVW and axle capacities at
http://www.oldgmctrucks.com/photos/51_SpecsAllModelsG.gif
