Many Iver Johnson and U.S. Revolver Co. revolvers have a visible serial number on the trigger guard, under top strap, or bottom of butt. However except for earliest guns, the "real" serial number is hidden on the grip strap under left grip panel (or one piece grip), with a letter prefix to identify the years made.
(Other details are also necessary, as the same letter prefix and number may appear on other models and calibers, and please start a new post rather than adding at the end of someone else's IJ or US question)
(Other details are also necessary, as the same letter prefix and number may appear on other models and calibers, and please start a new post rather than adding at the end of someone else's IJ or US question)