I vote not a milk bottle cap. I've never seen more than 2 colors on a milk cap, but I've not got tremendous milk cap exposure under my belt. The paper stock does not look right to me. Where did you find it?
"When I was a kid all the presidnets came to town. They arrived on milk bottles. The bottles came from Magoon's Dairy in a wire basket carried by a classic 50s milkman with a flat-topped paramilitary hat and a white uniform. For a time the bottle caps carried pictures of the presidents, and of course, a boy's major goal in life was to collect all thirty-something images. Magoon's supplied a handy collector's wall chart with rows of empty white circles where I dutifully pasted the lids in place with Elmer's glue. Milk had cream in those days, and it was important to wash the back of the lids carefully or the whole chart built up a nasty sour smell..." From.