Don't want to send you someplace else but, you should also ask over here: http://www.shotgunworld.com. They might be able to help out... Lots of talk about the 870.
FlaChef said:. Is there any reason not to chop an inch off this as it can ALMOST hold 3+1?
I would think the piece of wood is the magazine plug. Its only purpose is to limit the number of shells. Hunting laws sometimes require a shotgun to hold no more than 3 shells. Modern 870s use a plastic plug that is easily removed to allow the magazine to hold 4 shells. With 1 in the chamber, thats 5 all together (gotta love my mathematical abilities...)