Your "R" in the serial number may be an "A". I have an "A31xxx" on a .38 S&W Iver Johnson Hammerless revolver which I thought was an "R". Yours may be a Second Model, manufactured from 1897 to 1908. "A" and "E" serial numbers are classified as antiques. Models with "F" prefix are not.
The .38 S&W Second Model is unsafe to fire with smokeless (modern) cartridges. Use black powder cartridges only, and only after having the revolver inspected by a competent gunsmith and passed as safe for shooting.
There is an excellent book on these revolvers: "Iver-Johnson Arms & Cycle Works Firearms 1871-1993" by W. Goforth. You may find more information on your revolver there.