The picture shows a front and back view of this Nice Old Enameled G-MEN Premium Badge. This is believed to have been a mail away premium from FBI Agant Melvin Purvis in the 1930's. It has the same front that was used for a watch fob premium, only this one was made as a badge. It is marked in the enameled center:
G-MEN
It measures 1-1/8" x 1-1/2". It is in excellent condition with almost all of it's original luster.