well, I will assume that your guy is an honest fellow - no reason to expect otherwise - and that he is telling you the truth as he knows it, but therein lies the rub: somewhere, the hole was tapped for something other than 1/4x28 threads. This could have happened at the factory (since the barrel was made by people, and people often do odd and inexplicable things) but, aside from a really weird joke, or a prank gone wrong, I cannot envision this as anything except a very remote possibility.
... or ...
someone other than your guy made a switcheroo, and was embarrassed by his poor workmanship and didn't say anything about it, and the whole thing was forgotten until now. in any case, I would venture to guess that the hole is at least 5/16.
curious to see how it turns out, but I'm pretty confident that there's a vent liner out there somewhere which will fit what you have and get you up and shooting your new rifle, which you will love: these guns are sort of like having your daughter date a paratrooper: you can throw out his Playboy magazine collection, demand that she never see him again, have his car towed and so on: you can kick him out of your house, but you'll never get him out of your heart.
(I ought to know- I was one)
:thumbsup: