[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)