The 1937 resto pack lists both 1/2- and 1-1/2 ton trucks, but no other trucks. The 1938 resto pack lists 1/2-, 3/4-, 1- and 1-1/2 ton trucks.
Based on that data I'd say 1938 was the first 1-ton Chevrolet. Or at least I'd suggest they have been made since 1938, perhaps a 1-ton was made earlier and the series discontinued for some time.
Some of this stuff happened before I was born...
