Simply put, if your gun came with a wood forearm, yours is a Mossy 500 forearm/action tube. If it's plastic....
If the action bars, the two bars that attach to the forearm, are riveted directly to the plastic, it's a Mav 88 forearm and you can't be replaced with an aftermarket one as is. You'd need to replace yours with a Mossberg 500 action tube to be able to use aftermarket grips.
If the bars on yours attach to a steel tube that the wood or plastic forearm is slid down onto and there's a 2 notch nut at the front that can be unscrewed, yours is a Mossy 500 action slide tube and aftermarket grips will fit as long as they're the right length.
This is a 500 slide tube. The bars attach to the tube, not the wood or plastic forearm.
This is a Mav 88 forearm. The bars are riveted to the plastic and there's no steel tube.
Length....newer Mossy's have a 6 5/8" long action slide tube....again, the steel tube the wood forearm slides down onto. But, pretty much all, all I've ever seen, anyway, aftermarket forearms are made to fit the older 7 3/4" action slide tubes.
Remove your forearm from the slide tube if you have a slide tube and measure the tube from the back edge of the solid ring the bars are pinned to all the way forward to the front edge of the threads. No nut. If yours is 7 3/4", you're ok.
Just finished measuring a few foregrips here in the shop. The ATI AR-type foregrip will fit the shorter tube (it has a molded in line where you cut it to fit) as will their "standard" grip. Their older Thompson style won't, but it can be trimmed to fit, too. Speedfeed won't, but it's almost identical to the Mossy standard synthetic and Mav 88 corncob so just buy a Mav 88 grip if you want that look. Ram Line won't. Pachmayr won't.
Hope this helped.
Peace.
richard