A picture of the ID plate on the left door hinge pillar would help. If the rivets holding the plate on don't look "right" it could be that the plate and the serial number do not belong to the cab.
Another question: I can't tell from the driver's side picture you posted. Is there a vent door on the outside of the cab forward of the left door?
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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