I'll reply since I'm still awake and on-line. I don't have enough experience with the Arsenal rifles to be sure if they welded on a muzzle nut over the existing threads or completely de-milled them. common sense would lead me to think an American AK mfg. simply welded a muzzle nut on the threaded bbl.. 99.9 or 100% of the true RPK pics I've stored simply have a muzzle nut but they do seem to me be a rifle that a flash hider would actually be installed on. Then again at full auto fire I'd have to question their performance since I don't have one to play with..
Your call on a muzzle device I'd say, the plain nut is very common. I see where you're at with the sunset of the ban and what-not, you've got threads and want to add something while the window's open. Depending on parts count, Global Trades has a nice selection of what they claim to be "experimental" muzzle brakes and flash hiders for the 14x1 AK's that are U.S. made. I'd buy a couple, choose my favorite and sell or trade the other when they too are banned.