Most of the years issued in our books are strickly guesses by the authors.
The only absolutly correct years are on Hubs, T's, harps, etc. Chips with records.
That said:
One member has the Burt records and is working on publishing them. If and when that happens we will have exact years issued on 90% of inlayed chips issued after the 53. Pre 53 Burt records were destroyed in a burglary/fire. They could not find anything to steal so they started a fire.