Ken,
My comments were based on what I could see from the photos as well as other published information. Regarding the link you cited concerning the 6.5x54, the commenter kuduae is well known to us and his opinion is to be taken seriously, and with deference. His posting of a photo showing the 6.5x54 cartridges side by side, show pretty clearly that a 6.5x54 Mauser chamber will not accept a 6.5x54 M-S cartridge. In my comment, I mentioned manufacturing tolerances,but the differences shown in kuduae's photo are much too great to be covered by that. Consequently, I have to believe, now, that you have a Mauser rifle chambered for the Mannlicher-Schoenauer cartridge, as unusual as that is.
Mike