[giles]

FWIW, I would expect the likelihood of engine serial no. (MUN) errors to be low. The Pontiac engine plant only built Pontiac V8s so the MUNs were always in the same location and sequential. The vehicle VIN stamps on the block were added at final assembly just prior to the engine placement in the chassis. At all plants there were a variety of engines and / or chassis (Norwood for example had Pontiac V8s, Chevy 6s, small blocks and big blocks, at VN it was even more varied as they built both A and F cars). The probability for variation in location and misstamps would be much higher at final assembly, as locations varied alot as well as numbering sequence, as cars were not run down the line in exact VIN sequence in those days, like they are today.
As to this vehicle in question, it looks odd that the last two digits were restamped, and very poorly. The "original" stamp looks to be "29N122918". Now if this were the actual original vehicle, the odds that an owner of "20N122912" would later come across the 29122918 block and restamp it (so poorly) are about NIL. Probably an in plant screw up but I have never seen one done so poorly, they are usually restamped with a simple restrike, but I try to never say "never"!