I carried one of the Marine Wingmasters with the bayonet lug when I was on guard duty at MCAS Yuma. All the examples we had looked like some real frankenstein jobs. Really crappy lookin stuff. Granted looks aren't what it is all about, but still. It is possible they were just beat up relics left over from before the switch to Mossy 500s.
I am trying to get my 870 shorter, not add more length up front, so that isn't I would be interested in for a bayonet. On the other hand, it might make a good accesory point for something else.