Have you taken it out of the stock? If it's European it will have proof marks underneath the barrel. So I'm still thinking it's related to the Ward brand shotgun I linked, that knurled nut underneath and the square magazine are pretty unique. Since Montgomery Ward didn't make their own guns it's probably a rebranded Mossberg or Stevens.
Found a picture on a Gunbroker auction for a Ward Westernfield that looks very close to your action
Are there any levers, buttons, recesses etc in front of or behind the magazine?
Take a picture of the magazine area from the underside, including just in front of and behind the magazine. Then someone should be able to tell you if its detachable.