There are 3 books by Jon Speed. If you care to have a look into his "Mauser Archive", you will note that the "Six basic models" A,B,C,S,M,L and their designations were only standardized from 1922 on. As your rifle is apparently pre-WW1, you cannot assign one of the later model designations or descriptions! In the pre-WW1 era, Mauser made a lot of varieties, often to customer specifications, so long barrels were available on special order. On page 515 of the "Mauser archive" an early 10.75x68 rifle is shown, and on page 516 a long-barreled fullstock one.
BTW, standard B-Type foreends were uncheckered until the late 1930s!
You are right: Magazine assemblies of original commercial Mausers in both 9.3x62 and 10.75x68 are not only wider, but deeper too than those of the military 98s and their copies by FN and others. These larger mag assemlies, though combined with a standard-length action, require a deeper stock and a longer trigger, both same size as the Magnum action parts. To this deeper mag box a deep floorplate was added on 10.75s.