Old American Legion Pin with mark on the back - from my Grandfather

Most of you know I've inherited my Mom and Grandmother's jewelry when my Mom passed 2 months ago.

In digging through my Grandmother's stuff, I come across all types of military pins, some of which went on uniforms, some were awards/medals given for things my Grandfather merited. Ones like Bronze Stars, etc. I set aside as we have a shadow box where we put his uniform insignias , and medals/ribbons he was awarded.

Obviously he was also a member of the American Legion which is what this pin is from. I'm curious though as it seems very old and I found it very intriguing, particularly the mark on the back.

I'm hoping someone can tell me what time period these types of American Legion pins would have been given to their members and what, if anything, the number '66' shown on the pin means.