I think Mossberg uses more than one length of fore-arm tube on their pump guns, not just different forearms. Might be wise to check with someone who knows Mossbergs better than I do on that detail before ordering the part, or make sure the part you order fits the same size forearm tube you have now.
fwiw,
lpl
ETA: Yep- looks like senility isn't taking over quite yet. Take a look at the third picture down on the page at
http://www.havlinsales.com/model500.htm for the two sizes.