I'm guessing it is a single shot break open shotgun with an exposed hammer. Made sometime after 1900 and worth $50-$100 depending on condition.
Depends on what your shotgun is. If you have one of the older hammered shotguns (exposed hammer), you can decock them the same way you would any other firearm with an exposed hammer - you gently lower the hammer with your thumb. If you have a shotgun which doesn't have an exposed hammer, your only feasible way of going about this is to empty the magazine, be especially certain to ensure that the chamber is empty, and pull the trigger.
What isthe value of a Stevens double barrel hammer type 12 gauge shotgun Model 235 and when was it made?
I would have to do some searching to verify this, but I don't believe Stevens ever made a shotgun using percussion caps.
nitro hunter shotgun single barrel with exposed hammer stamped belknap hdwre and mfg serial 5516
The Marlin model 24 shotgun was produced by Marlin from 1908-1915.
If your pump 12 ga. has an exposed hammer, it's most likely a model 1898, as all later models of exposed hammer Marlin shotguns had the model marked on the upper tang behind the trigger. If it is not an exposed hammer, then the model designation is on the bottom, where the receiver meets the mag tube.
Model 49, 3000 made form 1925 to 1928 only
It has a visible hammer (or two).
Stevens used the Riverside name from about 1920-1945. It was a brand name used by Stevens for Montgomery Wards and other retailers. They still made some exposed hammer guns at least until 1940.
Riverside was manufactured by Stevens, so if you can identify the equivalent Stevens model, you should be able to find hammers at http://www.e-gunparts.com/ Or if you still have one hammer (or a broken one) you can take it to a local gunshow and dig through bins of miscellaneous parts looking for a match.