Coin finish offers very little protection from oxidation or other chemical reactions. In a lot of cases coin fin is nothing more than matte surfacing on the carbon steel base metal. Perazzi offered it in the past on some of their competition guns but when I enquired about it at my latest order I was told it was discontinued due to same. I doubt you'll get a definitive answer here on the culprit.... plastic degradation, VOC's from solvents, etc. unless you can positively identify the exact plastic in the case, chemical composition of the felt and adhesive, etc.
