If your gun has a serial number, you can always order a factory historical letter for it to see when & where it was shipped. Lee is correct on what these will sell for. Being NIB helps a lot. The S&W Police Riot Equipment catalogs show these guns, with prices, decriptions and the many accessoried that came with it, plus the various ammo. cartridges offered by S&W. Dummy display cartridges are available from cartridge dealers. These were offered in both the shoulder stocked version as well as the type you have.
An interesting tidbit concerns the famous Waco disaster at the compound where the ATF burned to death a number of men,women and children on the orders of Janet Reno. The talking heads on TV were interviewing the ATF PR people who were saying that the fire was set by the inhabitants, not the ATF's firing of these 37mm rounds into the buildings, as "no flamable ammo" was used. Video of the event clearly shows empty S&W 37mm teargas casings lying on the ground after they were fired. The casings have a warning on them " Danger- Flamable - Do Not fire into bldgs" Ed.