Sounds like an Ordnance Bomb, which is a mark of military acceptance, all right. As is the "US". But I don't recall ever encountering any top-break singles in military employ, nor are there any in my old Special Text from Aberdeen on Ordnance Equipment. Meaning that it isn't in the text or my memory, not that Uncle never bought any. I'd be interested in seeing the markings to verify they are what i suspect.