I picked this up at the flea market for a buck. It sort of looked like an Army Engineering DI until I noticed the metal loop on the end of the pin. It is a little smaller than a DI so I felt I had one of those unit lapel pins minus the chain and branch of service device. When I got it home I looked up CBMU 521 and found that it was actually a Navy lapel pin for the U.S. Naval Construction Battalion Maintenance Unit No. 521. It is marked sterling but the manufacture hallmark I can't make out.