[johnta1]

I would say it may be what John said:

A possible explanation, your engine was assembled and ran so poorly on natural gas in the engine plant, that your engine, originally destined for a 10B GTO, never got there and was plucked off the engine assembly line and set aside. Finally somebody fixed whatever was broken and put it back in inventory, whereupon it was placed back on the line and installed in your GTO in May. Whatever the case, it seems to be just an anomaly.

So, the ancillary equipment like alternator,etc would be a larger spread (newer) from the engine dates?
(instead of 10B dates it would be 05B dates)

[John V.]

Originally Posted by johnta1

So, the ancillary equipment like alternator,etc would be a larger spread (newer) from the engine dates?
(instead of 10B dates it would be 05B dates)

John, that could be. Or it could be that the engine was completely dressed out but not yet installed in the car (and partial VIN stamped) when the "defect" was discovered. If the latter, even the accessory items could be early dated. I'm not sure how complete the engine was before it was run on natural gas (I'm assuming they were still doing that in '68). But if not already dressed, perhaps what you suggest is true. Greg would need to tell us what he sees.

And my suggestion is pure speculation. It could be this YS simply got buried in engine inventory and grew old waiting its turn, in which case the dress components would likely show much later dates.. Luckily the PHS shows the EUN otherwise it would be easy to doubt the block.

Anomalies are interesting although often confounding in the absence of documentation. As PurelyGTO68 points out, documentation is a big help as it has in this case.

[RA1John]

PMD engine plant cast, machined, assembled and painted the engine.

The vehicle assembly plant "dressed" the engine...
Alternator
Starter
Coil
Fan
AC compressor
PS pump
Engine mounts

More likely that the engine plant held back an engine than the vehicle plant, though this was possible too if there was an issue.
Worse case scenario would be partial VIN stamped onto block ('68+), installed into vehicle, then discovered to have an issue requiring a new engine assembly. Vehicle plant repair wouldn't be changing engine internals. They'd pull another engine per Broadcast and stamp that with correct VIN.

Theoretically, it would possible for en engine to go from casting to install within a few days, especially if it went into the Pontiac assembly plant.