Thanks for the precise measurement. I couldn't be sure from the photo, but your button looked large than 1-inch in diameter, which is unusual. According to the button-book writen in 1976 by Alphaeus Albert, in the section on US marine buttons, in recent years the regulations of the US Marine Corps specify a 28.5mm button, That is larger than any from the 1800s. It is also larger than any of the "Fashion" imitations I've seen. You asked, "Trash or treaure?" I believe your find is an actual US Marines button, from the World War 2 era or later.