In old S&Ws, condition is the most important consideration in value. If the exterior is exposed to the elements, and the surface is not kept oiled or waxed, the finish deteriorates rather rapidly, and will eventually take on a brown patina, which is actually rust. Yours is headed that way. Not much you can do about it now, except to keep it from getting worse. The grips may be of a plastic called Catalin, and there are those who value Catalin stocks in undamaged condition. Regarding monetary value, and assuming that there are no mechanical problems and the bore is not rusted out, I'd think you wouldn't have much trouble in getting about $350 for it, maybe even a little more. You can shoot it if you wish, but use only standard velocity lead bullet loads. By the way, the "U. S. Service Cartridge" is the .38 Long Colt, which was the standard U. S. military revolver cartridge from about 1890-1911. It is a little shorter than the later .38 S&W Special cartridge, but otherwise dimensionally the same.