I thought you were going for a MAS36. Or both maybe?
For anyone unfamiliar with these, the tube under the barrel serves two purposes. The main purpose, something to fasten the bayonet too. The second purpose is to hold cleaning gear. This has been under dispute a bit on various message boards with people saying it is for keeping all sorts of various items. I have however seen a photo of a pull through and a wire brush that were kept in one so I'm sticking with that one.
The tube is easily removed by unscrewing both it and the cap. It has a large pin on the bottom which fits in a hole in the forend.