Riverdog, it is a smoothbore slug barrel.
Which, of course, came with Remington 700-style rifle sights. The heatshield is the standard metal job you can pick up anywhere. I milled a slot in it for the rear rifle sight.
The front end is somewhat harder to come by. It was from a boxed Remington military contract set intended for the Vietnam-era Remington 870 Mark 1 shotguns. It included the magazine extension tube, mag spring, and combination barrel clamp/sling swivel/bayonet lug. I bought one boxed set at the Cal Expo Gun Show in Sacramento back in 1997. It was only a partial set, missing the barrel clamp, but having the mag tube and bayonet lug bottom half.
I moved here to Florida, and just up the road from me in Merritt Island was a gun shop called B&H Guns. In their display case were several of the same boxed sets, all for sale. I snagged another one, and it was complete, yippee!
The set has a gold bead front sight threaded into the top of the barrel clamp. I ended up milling a slot in the clamp to fit the slug barrel's ramped rifle sight. After my modifications, it looks like it was intended to be that way from the start.
The bayonet is the standard M-7 from the M16/AR-15 series. It's funny how it locks on with the tip pointing upwards like that. Not a big deal, that's an extra-full turkey choke threaded into the muzzle, for my household 3" Magnum 000 buckshot carry loads. The patterns at 40 yards are TIGHT!
If you look at an original 870 Mark 1, it'll have the wood stock and forend. That's where I departed the norm, and went with the SpeedFeed black synthetic Monte Carlo set. It aligns the eye right now with the rifle sights.
Next addition is gonna be Meprolight inserts front and rear.