These are belt buckles. My dad collected celluloid, which some of these were made from, and I have dozens of them. They were made forever, and in every material imaginable, and every size. I have mother of pearl ones that are less than 1/2" wide, and celluloid ones that are 3" across. Some enterprising person probably saw then in the 80's and said aha, but these are old ones, especially the green one. They are probably both plastic, although the green one might be celluloid. They are no newer than the 50's. Watch for them, you could get lucky and find bakelite ones, which are quite valuable. The green one is probably more fragile than the black one. They aren't valuable, but they're cool, and we're probably used in all the ways you guys described. I'll never part with mine. I'm in my 50's, and dad had most of mine since before I was born. Just some fun info. The shirt ties if the 80's may have even been made from the same molds.
PS for some odd reason, the old ones almost always show up in button boxes. I never have figured out why, unless the buckles came with the outfits, with the belt, and we're salvaged when the buttons were salvaged, which is where most of grandmother's button collection came from. Waste not, want not was a common phase from my grandmother.