Sweet button find. Def like Dan said for time frame. Just a bit after your last eagle button.
A US dime is 17.9mm, so a 16mm button is just a tad smaller. Cuff or collar button. I'd only need to rely on calipers when it comes to something like bullets. CW, cuz many of the muskets I find have either hit something or been shot. Only the drops are round and an accurate measurement taken.