I'm not an expert on anything about the steel/heat treat of Mausers.I could have it all wrong.
My understanding:I do not expect a Mauser to to be hardened throughout the receiver.Most of the surfaces accessable to a hardness tester would not be hard.I would expect the body of a Mauser to be in the mid to upper 20's Rockwell "C".As is a typical 1911 slide /frame.
What they did was pack harden localized areas that required hardness.Locking lug areas,extractor cam area,etc.These small areas have a higher surface hardness
My questionable thought process:These old rifles were proof tested,and are now quite "experienced".They have fired thousands of rounds over the last 90 plus years,and pressure levels/bolt thrust are at least comparable.
So.IMO,unless some evidence of the bolt lugs setting back into the lug recess surfaces is evident...I'm not sure why it would suddenly begin now.
A question might be "How can I get an idea if the locking lug recesses are hard?"
Maybe this would help.The mold business uses core and ejector pins.There is a business "DME" that markets mold components.
A medium hardness core pin id Rc 30-35 through hard H-13.A High hardness pin is (IIRC) about 53-58? Something like that.Then ejector pins are nitride to a very high surface hardness,but the core hardness is 40-45.So,grind off about .050 of the end,and the interior tip will be Rc40-45.
These are probably less than $10 each
So maybe something like a 3/16 diacenter punch could be hand ground,and tested against the actual locking lug recess surface.If the Rc 30-35 pin won't leave a dink,its not dead soft steel.Rc 40-45 is pretty good receiver hardness.If its about even,blunt punch and small dimple,I'd call it good.If a high hardness pin bounches off...its woodpecker lips hard!!I don't think this hardness is necessary,but it would say something.
My point,just because a smith might notice drilling and tapping for scope bases is easier on a Mauser than a 1903 ,it does not mean theMauser has soft locking lug recesses.
One outfit that knows firearms heat treatment is Turnbull,but it may be high $