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
The best way to attach an M7 bayonet to your Remington 870 is to get S&J Hardware Bayonet Mount. It is extremely good and reliable, it is CNC machined from solid steel. Will work with Remington 870 factory magazine extension or S&J Hardware magazine extension for Remington 870.