Iver Johnson was in business for over a hundred years, at one time they were one of the largest manufacturing firearm companies in the U.S. Sometimes the only way to identify the model and year is by which patent dates are stamped and the location of the same.
Serial numbers for Iver Johns are in two location, one of the trigger guard and one stamped on the left grip frame under the left grip. The number under the grip will have a letter prefix if any. The letter prefix (* or the lack of a prefix ) may be able to tell us what you have. While you have the grip off, does it have a flat mainspring or coil.