That 'threw' me a bit, as well. Perhaps the author was referring to a custom ordered or rebarrelled Mauser? As I am not well versed on Mausers, I'd be guessing. Perhaps the clue lies in his term, "unique big bore concept"?

I do know that Mauser had several offerings (including raw 'actions') available by 1939 for 9X57 and 9.3X62 cartridges. Some images from the '39 Stoeger:

Oddly enough, when I ordered dies from RCBS in 1990 or so for my M1910, they arrived in a box marked thus:

This gave me pause at the time, I called them (pre - internet) and they assured me that they were proper for the 9.5X57 Mannlicher Schönauer. They make nice duplicates of DWM 531.

A rough scan, but these are, left to right, 1) 9.5X57 as ordered from Reed's Ammunition and Research, 2) My old unfired handload from .35 Whelen worked through the RCBS dies, 3) original (1926) DWM 531, 4) once fired from the .35 'reworks'.

The projectiles in the Reeds and in my own handload pictured are Hornady 3715 (.375 270 grain RN) which, unfortunately, seem to be nearly extinct these days.