That makes sense about the "nitro" proof.
Anyway, I went down to the shop, which has "gunsmith on site" on it's website. When I say I have a question for a gunsmith, I'm told "we can't help you there...". Nice.
So I can't make it anywhere at the moment, and decided to find out for myself. (Yes, I'm getting my work checked by a professional before I shoot it). I took a wax cast of the chamber. All of the dimensions from the belt recess and forward are within .003 of .300WM (according to the chart on the 6mmBR site), if I first take off .012 to assume that the wax shrinks as it cools.
Measurements between radiused contours were less accurate, of course. Despite this, my case neck measurement of .268 matches the .264 of .300WM, compared to the .318 of .308 norma mag. (I quickly ruled out .300 H&H, Wby, and RUM as well).
I tried to get a hold of the seller as well, but I'm pretty hard to get a hold of myself right now, and haven't been able to hear back yet.
I looked around on the net and found out the model. It's a 1900. IF you take this gun:
...and remove the sights, install weaver mounts, add a recoil pad, glossier bluing, and picture the stock with a richer colour, better grain, and more walnut-looking forend, that's the gun. It has a really nice lefty palm swell, I've never felt anything quite like that. After a few touch-ups to the finish and a decent scope, it'll be ready for the hunt! Somebody had a schmidt-bender at the show that would have been perfect, but too pricey for me. So was the rifle, I might run out of money soon and have to put it on the EE, but I'll avoid that at all costs!
Thanks again everyone for all the help!