Howdy
Around the turn of the Century Iver Johnson completely redesigned their revolver line for the new Smokeless Powder.
It is very easy to tell a 'modern' IJ from a Black Powder only IJ.
This is a Black Powder only Iver Johnson. First off notice that the little owl is facing forward. Next, notice the shape of the locking slots on the cylinder. Notice that there is only one straight edge to the slots. The bolt only bears against one side of the slot, the hand is pushing the cylinder in place when the trigger is pulled to keep the cylinder from rotating backwards. Lastly, if you pull off the grips you will see that the hammer spring is a flat leaf spring.
This is one of the later versions of the Iver Johnson. First, notice the little owl is facing backwards. Second, notice the shape of the cylinder locking slots. They have a hard edge both top and bottom, the bolt keeps the cylinder in battery, the hand does not enter into it. Third, if you remove the grips, the main spring is a coil spring, not a leaf spring.
Iver Johnson used better steel for the Smokeless guns. I don't care what the guy said at Cabellas, I would
NEVER fire an old Black Powder IJ with modern Smokeless ammo. There is more than just peak pressure involved here, the pressure curve of Black Powder is much slower than modern Smokeless, and will shock the old steel less.
From what I can see of the Original Poster's photos, he has a Black Powder gun, by the shape of the cylinder locking slots.