You've got a 3600. Cosmetically, the bed and frame are longer
than a 3100, with 3 stake pockets per side vs 2 for the 3100.
The 3600 (3/4 ton)has 8 lug hubs vs 6 lug for 3100 (1/2 ton). The 3600 is geared lower as well.


Steve

'49 3800
'52 3600
'57 3100