The DoD bought a lot of those Rem 870Mk1 versions decades ago.
They also bought the conversion kits, which include the magazine extension, barrel clamp w/bayo lug and sling swivel, bolts, and replacement mag spring. The kits were packaged in a neat little cardboard box with a big green "Remington" on the side.
About 14 years ago I found one of the kits, new-in-box, at the Cal-Expo gun show in Sacramento.
I did some reading after seeing a real-live 870Mk1 guarding a SAC missile silo in the Great Plains, and used a WalMart 870 3" Express model as the donor gun for my own 870Mk1 example. I used a Remington 20" smoothbore deer barrel with rifle sights to better match the magazine tube. The extended choke tube brings the muzzle just forward of the bayonet stud.
It's a heavy piece of hardware, probably intended to stiffen the barrel and mag tube in the event one really does use a bayonet to ventilate a bad guy. There are three bolts holding the assembly to the barrel and mag tube, which is why one will find a ring on Remington factory mag extensions for clearance of that offset middle bolt:
Some 870Mk1 shotguns had rifle sights, others had a simple gold bead front sight. I really like the tall front sight of the deer barrel, so I notched the barrel clamp to accomodate the sight. The metal heat shield on the barrel also got notched before parkerizing to accomodate the rear rifle sight.
In a departure from the original wood stocks of the military 870Mk1, I went with a synthetic. The stock is a SpeedFeed, sans shellholder, with Monte Carlo comb. When shouldering, the eye is in perfect alignment with the rifle sights, particularly that big front sight, which is the center of where the load of 00 Buck will go after it squeezes through the Extra Full Turkey choke. I enjoy tight patterns out to about 40 yards with the combination, entirely adequate for a defensive shotgun.
Not a bad tool for boarding parties, either. Note the young marine below, but I'd love to ask him what happened to the sights on the gun...
