Most likely there is a bolt lengthwise through the stock. You will need a long screwdriver to reach it, and may have to remove the recoil pad or buttplate first.
The recoil pad is usually fastened with screws inside the rubber. You will have to feel for the holes, as the rubber closes over the screws.
Remove buttplate. Use a socket extension and rachet to unscrew the stock bolt (it's in that hole in the center of the butt)
There are two small holes in the butt pad of the stock. Lubricate a Phillips head screwdriver into the holes and remove the butt pad. Using a long flathead screwdriver remove the screw in the center of the stock. To add new stock simply reverse.
Remove the butt plate. There will be a hole. There should be a slotted screw at the bottom of the hole. Use a long screwdriver and be careful to not damage the stock.
In front of the butt stock .
i have recently picked up a Boito .410 single shotgun it is break barrel where the trigger guard is the release for the barrel it is a Brazilian made shotgun imported to the U.S from the early 60's to mid to late 70's i seen a couple of these guns online but none near the beauty as mine has ivory inlaid in a diamond pattern once on each side of the butt stock and again on the front stock each side it is engraved in one side with the Boito. logo name and a design very amazing shape for a 40 yr old gun ....
The butt plate screws (2) at the end of the stock must first be removed. You will need a long shank flat tip screwdriver to remove the screw at the bottom of the center hole in the stock butt. Reverse to reassemble.
some have larger blade screws( 6 inches long) inside the buttstock, remove the butt plate and it will be recessed inside, you need a long, large screwdriver.
Type your answer here... Remove the two screws on the butt and you will find a single screw inside that holds the stock on. Remove the screw and the stock can be removed revealing serial number on top.
The butt of a rifle or shotgun- the part that goes against your shoulder.
Mossberg 500.
Remove the butt plate and you'll see a hole. Down the hole is a nut on a threaded rod, holding the butt onto the gun. Remove the nut = remove the stock.
Best left to a gunsmith