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I do believe the "GMI3" marking is the maker's mark. It is a link style with a total of 7 hearts that are interconnected with a small "o" ring in between. As the pictures show, these hearts are mostly flat but do have slightly upraised edges when seen from the backs.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.