Yeah, they're basically akin to hens' teeth, these days.
The Mossberg 590 is an easier one to find, or the Norinco copy of the Winchester 1897 Trench Gun.
The Remington 870 Mark 1 was a Department of Defense variant that saw a lot of use with USMC boarding parties. That's where I saw my first one. I *stumbled* into a couple of the factory Remington boxed conversion sets, one of which found it's way onto my WalMart 870 Express Magnum. M16 bayonets are an easy part to find, in retrospect.
Pluses? It's hellaciously stout, the 2-piece mag extension tube/barrel clamp is absolutely beefy, with fairly thick steel used in it's construction. The barrel and magazine extension are basically unitized when the three clamp bolts are tightened up. Slug accuracy actually improved! The gun's heavy - close to 9 pounds empty. Loaded up with my home load of 3" Magnum 000 Buckshot, recoil is actually quite mild. And the bayonet snaps on quite securely, obviously serious bayonet use (!) isn't going to cause the shotgun to shed, break, or bend parts. That explains why the front end is so stout in construction.
Minuses? Did I mention the gun's heavy? It's no longer a fast-pointing upland field gun for pheasant, although I could use it for that if I need to. If I want to remove the magazine extension tube, that heavy clamp system has to come off. The bayonet does NOT remain attached when sitting ready in our bedroom. I have no problem using the gun to keep us safe in our home. It proved itself when I lived in a bad neighborhood in California. But I'm not so certain a district attorney would look the other way if an aggressor got his innards stirred up with a bayonet attached to the shotgun. I'd already had 8 chances to stop the attack with the buckshot.
Here are the obligatory views. Note the USGI green canvas sling. I like quiet slings:
Outside, with blade attached:
Close-up:
Close-up w/blade:
I'll try to dig up some images of USMC boarding parties in action using the 870 Mark 1 later...