I have two M15A4's. Both came with a "pinned" brake installed. One was post ban, pre repeal, the other post repeal, but all they did was throw a flash hider in the box and left me to my own devices to get the "brake" off, and the flash hider on.
Good news if you have a pinned brake is, its only about an hour of filing, and the new muzzle device goes right on.
Since yours isnt threaded, maybe give Armalite a call/email and see if they can help you out.
Both of mine have been good shooting rifles, and reliable. My only complaint is, both are mag sensitive as far as fit go, and you need to try and verify that the mags you want to use will fit. Not all do.