There were the following SB manuals for the 15 and 20 CWT GS Trailers:
SB-4 Gas Welding Trailer - 1943
SB-4A Gas Welding Trailer - 1944
SB-5 15 and 20 CWT. General Service Trailer - 1944
SB-5A 15 and 20 CWT. General Service Trailer - 1945
There were subtle differences in the manuals although I'd have to go and fish them out to determine exactly what they were. One of the most obvious differences between 5 and 5A was the illustrations and I have attached one as an example.
