101= 1/2T short, next letter can be engine or assy plant, engine is either blank (6) or 8 for V8 (no 9's), if it's assy plant it's C, P or S (no 9's).... If I wanted to go out on a limb, I would say it's not a 9. The S is correct for 1958/1959 and the rest is sequence serial number. Have not seen "A" at end of USA GMC. Have owned 3. You say it's a USA vehicle and you are sure it's not Canadian.
Canadian GMC's did have different numbering and they DID have "9" for 1959 and they DID have "A" at the end for GVW less than 6000. But they didn't seem to start out 101 as USA did. So misread, misprint or Hybrid number/hybrid truck. What wheel base is this Panel? Should be 114". Is serial plate on driver hinge pillar or outside firewall or just on Title?
