Yes, it is either a close copy of or a surplus bayonet adapter/magazine extension brace that was a part of the Marines Remington 870 Mark I shotgun. A Remington 870 with a 2.75" chamber, walnut stocks, parkerized finish, +3 magazine extension (Remington made and still available), 21" barrel, Williams type rifle sights, and that bayonet adapter/mag extension brace, which also had a sling swivel mount. Remington Arms had made the mount available to law enforcement agencies in the 70's, but I don't think they had it available for long.
Numrich Arms (formerly Gun Parts Corp.) had either stripped or semi-finished ones about 8 years ago for 15-16 dollars. They did not have a rifle front sight on them, but rather a bead sight (though photos from the Persian Gulf War show Marine boarding parties equipped with bead sighted 870 Mk.Is). Nor did they have the bayonet adapter/swivel mount attached or available. And one of the holes drilled in the side was not put into the right spot to fit with the magazine extension tube.
When Scattergun Technologies was owned by Roger Small, there was a Military model of their 870s with a bayonet mount/adapter/brace, and the part was available with an attached heat shield (specifically to fit 18" barrels). The part was around $100 back in the mid-90s. However, when I contacted Scattergun Tech, Bill Wilson had acquired them, and they had no such item available. I too am looking for a couple of them, and have had poor luck (should have grabbed the Scattergun Tech ones when I first saw them in the catalog). It is a hard to find item.
