I can't offer any help on which MS units might have been issued these, but I do have an anecdote; in one of his obits, there is a story that Captain R.T. English (of English Battery) while at Ellis Cliffs in late '63 or early '64 used a Maynard carbine (or rifle) to shoot a USN officer on one the ships in the river near there. I have no other evidence so I have no proof this happened, but it's a pretty specific reference.