I looked through those specs from 1941-1954. It appears that the 2 ton came about post-war with the first mention of "1 1/2 Ton Special" in 1946 specs. The only explanation of the term I saw was in the 1950 specs (page /5051.htm). This states that the only difference between the 1.5 Special and the 2 Ton is the identification plate showing 15000 max GVW. The plate is an option - RPO 402. I am thinking this must have to do with a government regulation for weight. If I can find more, I'll post it.
http://chevy.oldcarmanualproject.com/chevyresto/5051.htm
