Could be a whole body swap onto a 4100 or 6100, not at all uncommon.
Another option is the frame was shortened, any welds in it?
Looks like you have a single speed rear axle which is most common in the 1.5 ton, so I'd lean toward a 4100. To confirm or deny look up the frame height and thickness specifications and compare to the 1.5 and 2 ton 137" wheelbase trucks. I thought it was in my shop manual for 48-52 and it may be, but I couldn't find it in the 48-51 manual here
http://chevy.oldcarmanualproject.com/ might be in another manual or book on that site.
Grigg
