No facts, but here's a theory ...

Your worn 25c C&J mold chip (listed as 1980's) sure looks similar to the purple 25c C&J mold chip (listed as 1960's). Do we know for sure that yours is 1980's (not 1960's)?

If older, that would be during the time that The Mint leased out certain games to other operators -- Pan games & poker games, I think.

If used at a leased poker game in the 1960's, maybe it was used only briefly at The Mint, then the operator left and took his chips to another location and used them for ante chips -- for example, re-purposed at a small California poker room, or something similar. As ante chips, I've seen some pretty worn chips like yours.

Again, just a theory, but I would double-check the decade it was used if records are available -- 1960's vs. 1980's?

Heres' the 2 chips for comparison (Pics from MOGH/ChipGuide).