Cracked the code / simplest description - NcSTAR ShotGun Rails Bayonet Mount Remington 870

If your 12 gauge mag tube extension diameter reduces from 1" to approx. .8" at the front end, order the "Remington" model. If you have the standard 1" mag tube extension that probably 90% of people are using, order the "Mossberg" model. They are not brand-specific, just very confusingly marketed.