On the 500 the mag tube is capped with a solid piece of steel with a hole drilled and tapped through it (3/8", 16TPI I believe). The barrel retaining BOLT threads into this hole. On Remmingtons and others, they use a barrel retaining NUT that threads over the mag tube. This lets you put an internally threaded tube (mag tube extension) on in place of the barrel retaining nut. I'm not sure how clear that is. Maybe somebody has pics to illustrate this?
EDIT: just saw your pics, hard to tell from them, but that really doesn't look the same as the 500. The 500 uses a barrel retaining bolt, not a nut.