If you have a 1951 short bed GMC frame, it would be a 1/2 ton model.
A is a notation of bearing set dimensions, there are a few different sizes A B C etc. . . .
The "letter" preface in the Engine Serial Number was used to indicate a variety of possible combinations of "non-conformities" to "standard" engine specifications for cylinder bore and crankshaft journal/bearings specs:
"A" preface specified:
+ 0.005" cylinder diameter bore
- 0.010 main bearing journal diameter
- 0.010 rod bearing journal diameter
