Here's the deal on the 500...
Older 500s and any that use the standard action tube, are GTG. Those that have the newer tri-rail or integrated action bars and for ends like the maverick need to purchase a compatible action tube assembly and nut in order to use the forend.
We list it for 590/590A1 only, since with a 500, you may or may not need additional parts for various versions.
The forend also fits the 835s we have tried it on, but we don't guarantee it will work with all configurations,