The series numbers for '56-'59 up thru the 6000 series are the same "tonnage" as the previous years.
31-32-34-3500 1/2 Ton
36-3700 3/4 Ton
3800 1 Ton
4000 series 1-1/2 Ton
5000 series 2 Ton LCF*
6000 series 2 Ton
*LCF=low cab forward, formerly known as COE.
In 1956, Chevy added even bigger trucks, which I believe are as follows (someone please correct me if I got the tonnage wrong):
7000 series 2-1/2 Ton LCF
8000 series 2-1/2 Ton (Conventional)
9000 series 3 Ton LCF
10000 series 3 Ton (Conventional)
Note that MikeB's lists also show the "special" and "heavy duty" models. I'm not sure how these designations affect the nominal rating of the trucks. For instance, for the AD trucks, the 6000S series was called a 1-1/2 Ton Special, rather than the usual 2 Ton. Something similar may or may not apply to the TF special trucks. Heavy Duty options may increase the actual GVW without affecting the nominal rating.
