All the Mossy 500 series pump shotguns use the same buttstock. That's the Mav 88 and Mav 91, the Mossy 500, the 535, the 590, the 590a1 (both 5+1 and 8+1), the old model 600, and the 835.
All of those also use the same style forearm hardware and all are interchangeable. The Mav 88 and 91, though, use riveted/pinned one piece forearms where the action bars are pinned to the solid forearm plastic....the others use a steel action slide tube that the forearm plastic/wood is slipped onto. To use a Mossy 500 forearm on a Mav 88, you thus need to replace the Mav one piece with a Mossy 500 steel action slide tube. On the same point, though, some tactical Mossy's, the ones that come with factory plastic, are sometimes showing up with Mav one piece forearms these days.
The innards of a 500, a Mav 88, the 590 and 590a1 are all identical and can be swapped at will. The extended chamber guns, the Mav 91, the Mossy 535 and the 835, however, use stretched parts so only those can be subbed for those.
Mag tubes and barrels; All Mossy and Mav mag tubes come in two basic flavors; the solid take down screw tube 5+1 500 and the cap tube on the 590a1 and the 835. All the solids are interchangeable within their capacities and all the cap tubes are interchangeable within their capacities. You can use a basic 500 solid screw mag tube on the Mav 88, the 500, the 5+1 Persuader etc. You can use the 835 cap tube mag and barrels on the 835, the 5+1 590a1, the 5+1 capped Cruiser etc. The dif mag tubes can be freely interchanged between all the models as long as you switch to the appropriate barrel thus the 590a1 barrel and capped mag tube can be used on a Mav 88 as Pilot mentioned. If you have an 835 and want to make it into a great tactical/home defense gun, the barrel off the 590a1 5+1 fits it as is and a 500 tube and barrel combo fit it as well....and the 590a1 and the 835 (capped tubes) can use mag tube extensions.
The trigger groups are all the same basic part between all those guns, the Mossys have the safety built into the receiver while the Mavericks have the safety on the trigger group itself, and all except the Mav 91, the Mossy 535 and 835 are interchangeable. A Maverick 88 (trigger guard safety) can be used in a Mossy 500 and the trigger group safety will work just fine....but if you use a 500 trigger group with no safety in a Mav 88 receiver, you'll have no working safety. Some sporting shooters used to trigger group safeties and some tactical shooters who use pistol grips on their 500's have switched to Mav 88 trigger groups in their guns just for the safety. You can buy self adhesive aluminum blanks that take the place of the original 500 receiver safety to cover the hole.
Did I miss anything?
Be safe.
rich