Here are a few differences shown in the 1955-2nd Series Truck Shop Manual Supplement and in the 1955-2nd Specifications Manual:

- chrome plated upper piston ring in 235 Loadmaster and 261 Jobmaster

- silchrome XCR steel exhaust valves in 235 Loadmaster and 261 Jobmaster

- positive rotation type valve rotators in 235 Loadmaster and 261 Jobmaster

Pre '68 Dave might come up with more differences?