As far as I knew, a 1951 3800 1 ton truck only had a 14 inch rear brake option
If you found 12 inch drums on the back end then it is not original equipment.
Edit:
5.809 drum assy., brake, front and rear
47-52 1 Ton front SRW 3847129 (12 inch ID - 8 stud)
47-52 1 Ton front DRW 3847698
47-52 1 Ton rear 609808 --> 3754313 (14" brakes) (as of 1959 part substitution)
according to the parts manuals, the 1951 3800 1 ton front and rear drums are unique to that model size for 1947-52 years and not shared with any other vehicle.
these drums are not reproduced and pads are getting more difficult to find every day.
-s
