Iver Johnson Champion 20 ga shotgun

Many years ago my grandfather found a shotgun that had the barrel sawed off and the entire shoulder stock was missing. It was found in some rocks along the cliffs where he used to live. He cleaned it up and fixed it best he could. He died almost 20 years ago now and I finally decided to see if I could find out when it was made.

All I can read on it is "IVER JOHNSON ARMS AND CYCLE WORKS, FITCHBURG, MASS, USA" and "CHAMPION" below that. All on left side of gun. On the top of the barrel it says "20 GAUGE CHOKE BORE BARREL AND LUG FORGED IN ONE", below that it says "PAT'D JUNE 15 15 PATS PENDING". I have found only one number of any kind on it, on the metal piece behind the trigger that the trigger guard it screwed to it says either "81459 A" or "31459 A". This looks like it was stamped on by hand because the numbers and the letter are all crooked and uneven. It has the breech opening lever on top of the handle area and the hammer is there on top also, in front of the breech lever.

I am hoping that somebody can help me. I have been curious about this gun for a long time, I just never had the time to dig into it before now.