Ok, I see what you mean. I can't see details in the picture but from the research I have done, based on all the published material I have read, Mauser never made a C-96 like this. It could be a Chinese copy or it could be a C-96 that a master gunsmith altered. These are the only explanations for the details you described. I lean toward the latter, as I cannot see details in the pictures that may reveal a Chinese trait. You state that it has not been "monkeyed with" but I have see other long barreled examples that the workmanship is superb. Mauser did make some carbines that had longer barrels but they had other traits that would be noticeable. The chambering in 9mm also points toward an altered original. This may not be the answer you were looking for but it is the most likely explanation. All this said, Mauser did some unusual things or custom orders that have not all been documented. In my research I have discovered a few. So if you are able to have it x-rayed or authenticated by Mauser documentation I would happily stand corrected.
Regards,