In putting all the accessories back into the cover of my URM-25D sig gen. Everything is there except I find that I'm missing the UG-684A/U connector adaptor. It is mounted inside the front cover and is attached to what looks to be a dummy PL-55 male plug. Might anyone know what this thing really is? Is it, perhaps, simply a phone jack to BNC adaptor? Never have used one so am curious... TNX