14K Unknown Keystone Society Pin Made by CH. Elliott

I got this from an estate sale along with many more items. It seems to be a keystone pendant in 14k. Not sure if it is a society or fraternal order pin and I'm not sure how old it is. It has 21 on the front and M T H also with the t underscored. The estate I got this from had a few Masonic pieces as well as some Knights of Columbus stuff. Does anyone know if this is a society pin?