@Brian Rothhammer
It is a curious thing . The 10.75x68 mm Mauser offering is far cheaper than the other three offerings .
I noticed that, as well.
My first guess would be that the 10.75X68 may be built on a shorter or less 'beefy' Mauser receiver than the .404 Jeffery, .280 Ross, or .318 Westley Richards Express options. Surely a Mauser expert could verify or refute.
Images from 1939 Stoeger catalog (US):
Note the raw 'action' for a 10.75X68 (#13 below) is listed at $80. USD while the .404 Jeffery, .280 Ross, and .318 Westley Richards Express are each listed at $100.
The Type A 10.75X68 (Model 627 below) is, indeed, priced far below cost of the .404, .280, or .318 rifles. The paragraph below does state, however, that "Models 627 and 628" are built on "the famous Mauser Magnum Actions... [sic]".
The pricing of the listed Type A in 10.75X68 (Model 627 above) is more in line with the Type B offerings listed below.
A 10.75X68 Mannlicher Schoenauer ordered from the same catalog cost the purchaser an additional $15. over the other "High Velocity" (now commonly referred to as 'M1925') offerings.
Here's a handy 'inflation calculator' to convert 1939 USD to today's equivalent:
https://www.usinflationcalculator.com/