Essentially yes they will. I had a Pardner Protector, I put a side saddle, hogue over molded stock, 2rnd extension, and sight rail all made and listed for an 870. Had no issues with fitment a all.
As far as the Mav 88 goes, nearly the same in respect to the H&R / 870 scenario. Almost all parts for a Mossberg 500 will fit. Only the safety location and trigger assemblies are different. 500 has the safety on the receiver, 88 on the trigger assembly, so they really don't swap well.