Remington 870 USMC Mark 1 (MK1) Bayonet Adapter is one of the interesting but very rare items. I tried to find one without any success. I know that they were available at online auctions but I can’t find one for a long time.
Special offer from S&J Hardware: Remington 870 +2 & +3 Round Extension and Bayonet Mount Kits
Remington 870 Bayonet Mounting Kit with +2 Magazine Extension for just $148.74
Remington 870 Bayonet Mounting Kit with +3 Magazine Extension for just $165.75
I have received several updates from J.R. and will be publishing them in the nearest time. And first upgrades are Remington factory +3 mag extension, an S&J bayonet mount and an Ontario M7 bayonet.