Remington did, in fact, manufacture them. You won't find them at WalMart or Gander Mountain, but here's your proof:
They were issued to the military as the Remington 870Mk1 variant, both in complete gun form and as armorer's kits. I managed to buy two original factory Remington kits, hence my 870Mk1 above and below.
DaveMc is right. Shotgun barrels are quite thin, and if you're going to hang a bayonet on one, it had best be a seriously stout mount, like the older Winchestr Model 97 and Model 12 trench guns. I've tried the slug grouping thing with my 870Mk1 and a rifled Remchoke, and the point of impact does not change much with bayonet hanging or absent. I'll give Remington credit for thinking their design through and making sure a few bayonet thrusts into a bad guy won't bend the barrel. Take a look at the mag tube and the long 3-bolt clamp unitizing everything, actually stiffening up the barrel. You will, however, notice a nose-heavy gun for just a 20" barrel.
