I can understand why this would be confusing, it is to me. I think there are considerations that make it come together, as Mills states manufacturing companies were playing around with configurations of which some were standardized, remember the gun is well over 100 years old. With that said there are actual examples of 11, 14 and one of our most knowledgeable collectors is actively looking for a 13 gauge. The breech loading shotgun was a new thought as Parker began production, there was the brass shell, the paper shell and in the case of your gun the bore was bored to accept a shell which didn’t make the standard list. Nothing wrong, just a part of the evolution of a very fine and honored product.

Jack,

No you did not upset me. The "gunsmith" at Greentop probably knew zero about early Parkers, for if he did he would have measured the bore dimensions at the least and told you it was a 12 gauge bore. He probably does not have a bore gauge so one must question any gun you buy from them. They advertise that they "buy collections," and that is probably for quarters on the dollar. I know the guy who sold it to you also claimed it was a 14 gauge since neither a 12 or 16 shell fit the chamber. But the key was that the chamber dimensions that Mr. Hoover gave you the dimensions for are perfectly correct for the 12b shell .

As far as Greentop, the guns they sell are always way overpriced as if there is some hidden gold in them, yet they offer a fraction of what they are worth. I once took a 100% restored Parker to trade on an 80% Trojan, and they wanted to give me one fourth of their price of the Trojan for it. The only guns I have ever bought from them were two broken Sterlingworths and even they were overpriced but I knew I could fix them. But they have to pay a lot of overhead. The best way to sell your gun is to pay $40 to become a member and post it here with good photos and a reasonable price and it will sell I am sure. That way you will get full market value. Someone will want to add such an early gun to their collection. That gun sold for quite a bit of money at auction before you bought it after it had been traded in. I believe if you Google the serial number with Parker Lifter you may still find what it sold for at that auction. The only thing to be upset about is two different people who deal in guns for a living have twice told you it is a 14 gauge, causing you to not be sure what you have.