Yes 1/2T short, 6 cyl, Pontiac, MI, 58274 should read S8274. (101 P S8274) the 7273rd off the line, I believe they started with 1001 or 1000.
The "A" is an anomaly. I do not believe the plate is original. I believe the plate was redone by someone. You can buy those plates. They misread something somewhere. and stamped it wrong. I hope your friend has a Title in their name and I hope that if it was reconstructed, it was done through the DMV. If so, they would issue a new plate in most cases in U.S.
If I was looking to buy it, I would be very concerned about the documentation. Like, why is the plate off the truck? Why does it not match the papers, etc. At any rate, I guess all you want is to cypher the serial. It is not altogether a proper number as far as I can tell. I have seen Chevy trucks with the "shadow" of the number behind the plate where they stamped the plate after it was installed, therefore indenting the pillar metal also. You may want to look there, GMC may have done that as well.
