I think your test is inconclusive. I tried a 30-06 round against the muzzle of a couple of 03's and get about 1/16" between the muzzle and the top of the case. But that could be due to a difference in how the muzzles were crowned or muzzle wear. I found no difficulty in chambering round with the action removed from the stock but that could be to grease in the chamber.
I see that you have a digital trigger pull scale, do you also
have a vernier caliper that you could use to measure the bore
diameter?
I took at a look at some of the other rifles that your seller was peddling, two looked very similar to yours in terms of stock shape. One had been chambered in 275 H&H the other in 300H&H. So maybe yours is also chambered for some British cartridge. A chamber cast maybe the way to go.
