I knew they existed, but I have never seen a field gage, for any chamber other than a military round. I am not sure a field would be typical, but a generous chamber on a lever gun, is not unusual since the lever has little enough power to close an oversize round. Dont forget, Win was going through QC problems in those days much like Remlin is now, (and most every one else)
I have heard that the term field gage, came from when a military rifle came back to the armory, it was to be taken out of the field, if it accepted that gage. Not certain that makes total logic, but thats the story generally told as too why its called "field".
Headspace is a complicated topic, and a bit misunderstood. Since the 30 30 headspaces on the rim, i would think you would find primer backing out a bit if you fired that gun. Not dangerous unless in the extreme, excess headspace becomes an issue in reloading, more than shooting factory ammo. Again, thats in a typical situation. All sorts of tests have shown, that the round will simply expand out to the limits of the chamber walls. Now if that round is full length resized, and fired, it then generally splits in the chamber, rarely damaging the gun, but a pain in the butt still.
Gun show deals are often final, but you are between a rock and hard place. I would take it back too the guy you bought it from, with the gage, and try to get your money back. There is no easy cure for that issue, seeing as too how Win is out of business. An oversize bolt, if such a thing exists, or a different barrel, or that barrel set back and re chambered, with all the other mods needed to make that possible, mag tube issues etc.