I've got an old Stevens single shot .410 sitting in the gun safe, it's a great grouse gun. I'm almost willing to bet your Iver Johnson is of the same manufacture as my Stevens as many of those old shotguns shared the same lineage.
I would suggest not actually making that bet. Iver and Stevens were different companies at this time as was Savage. The latter two combined later on but Iver was on its own until going insolvent in the 80s. The name was bought and sold a couple of times since. I think it might be part of the Kahn conglomerate now.
That link is about the best reference for IJ shotguns, the only way I have of narrowing down further is based on the opening mechanism. I don't think there is a data base for determining the year of manufacture for shotguns as with the handguns but I wouldn't be surprised if there was something in Bill's records. It is sad he died when he did, there was so much more to learn of these utilitarian guns.
