I was told probably 55 years ago that it would. Seems to me to be not a smart thing to try. If you compare the two assemblies it appears to me there are a number of differences. It would be a “ hold my beer” moment if someone did try it.
shade tree gunsmithing 101 at best. to this day, I never understood the attraction to want to make something work, just to try it. guns? bang? just replace broken parts with the correct parts. its not like we all are on a deserted island and must do something to keep a tribe alive.
Technically they’ll both fit and sorta function but installed in a m1100 the 870 trigger won’t lock action open at any time. The m1100 unit installed in a m870 will lock bolt open every time you single load it. Neither is a good situation