Congrats on your purchase!
Your VIN crunch is correct. The RJ prefix ('48, 1.5-ton, 137" wheelbase) is a 4100. There was no exact series identification on the exterior of the '48 trucks. The hood side emblems contained the word Chevrolet, along with the word Thriftmaster (small truck) or Loadmaster (big truck) to indicate the load capacity group. In '49, they scrapped the Thriftmaster and Loadmaster and added the exact series number under the Chevrolet. They still used those names for the engines, though.
The big trucks (1.5 and 2 ton) had the hand parking brake, as opposed to the foot pedal.
It sounds like this truck has the deluxe cab option, with the 5 windows and the stainless window trim. Is there also a driver's door arm rest and a right-side sun shade (visor)?
The engine is probably a '54-up 235, though could be a 261 too. We'd need more clues to identify it. Try to find the engine serial and casting numbers.
This truck should also have the exterior frame-mounted gas tank. The in-cab tanks started in '49.
Good luck!
