For 67 San Jose Mustangs I recall most often finding the ones marked R but there were other suppliers like you found. As they likely filled the containers that held the screws at the individual stations where these gauges were preassembled they likely just dumped more in with the last of the earlier batch creating a mix of both previous and current suppliers which would create a mix of different head markings at that station for some period of time. In other parts of the car plant containers with the hardware were often replaced with new containers so this reduced the amount of mixing of different suppliers and the older batch was often poured on top of the newer one making it easier and more likely that the "earlier" batch would be used up sooner in the assembly process from my understanding and discussions with some of the workers.
IMHO I believe that if you found this mix I would reuse them again unless you have a reason to question their originality. Was the car restored before you did? Was the gauge housing restored some place in the past? Things like that might make one question details found
