This is a vintage gold filled GM Diesel Service Craftsman Guild Member pin. This dates to the early 1960s, as GM diesel became known as Detroit Diesel in 1965.
The post on the back of the pin has some dents on it which makes it a bit hard to put screw the nut onto it, although the nut does go onto the post and stays in place.
The pin is hallmarked 'GF' on the back.