As far as I know, from several books ( "Bolt Action Rifles" by DeHass, and "Book of the Rifle", forget author, "Smith", maybe" ) the Norwegian Krag was originally chambered in 6.5X55, same as the Swedes. Both countries adopted the chambering at about the same time, but of course on country went with the Mauser, the other with the local-designed action. I would say that yours has been either set-back, and rechambered, or re-barrelled to what it is stamped. I think before I shot it I would do ( or have done ) a chamber cast to make sure.
It's kind of strange, I can't think of a time in the past when the M.S round was more popular, or available, then the 6.5X55, so why would one re-chamber or re-barrel to that caliber?? Maybe a past owner got caught-up in some of the old glory stories of the M.S. when it was used by "gentlemen hunters', who wrote the praise of the M.S. ?? Back around 1920-1940 or so??? I just re-read your post, you state the barrel matches the reciever ( serial #??). You know what I think, ( just a W.A.G., but it makes sense ), I think when it was sporterized, whoever did it wanted to stamp it with it's caliber, and simply stamped the wrong designation. Again, a chamber cast would be the ONLY way to go. Let us know what you find/figure out.
Mark