Howdy
Around the turn of the Century Iver Johnson redesigned their revolvers for Smokeless Powder. It is easy to determine whether yours was designed for Smokeless Powder or if it should only be shot with Black Powder.
This is a Smokeless gun. The dead giveaway is that the little owl on the grips is facing backwards away from the barrel. If you remove the grips, the hammer spring will be a coil spring. Also, study the cylinder locking notches. Notice how there is a hard edge to both the top and bottom of the notches. This means the bolt securely prevents the cylinder from rotating in either direction.
This is an earlier Black Powder Iver Johnson. Notice the little owl is facing forward, not backwards. If the grips are removed the main spring will be a flat spring, not a coil spring. Lastly, study the cylinder locking notches. Notice there is only one hard edge. The bolt only prevents the cylinder from rotating in one direction, the hand is what prevents the cylinder from rotating backwards, not the bolt.
Those are the keys to identification of a Black Powder Iver Johnson vs one that was built with better steel designed to take the pressure of Smokeless Powder.