The case, as applied in case hardening, is primarily a wear hardness surface. Case is still applied for modern alloy steels, such as nitriding, salt bath, etc for wear hardness. The habit of grinding off case layer in the shooting community to make parts "fit better" is a total folly and is an example of how foolish ideas become doctrinal behavior within a society. Once that case is removed you are down to a soft metal surface that will gall, wear, etc.
Case hardening is not meant to effect the heat treatment nor the "through hardness" of the steel. Carburizing requires hours of exposure to a carbon rich environment. Pre WWII it was common to pack parts in boxes that had bones, leather, etc, and heat the box and parts for hours and hours to develop a case surface. Around that time other more precise methods were developed, but all involve exposing steel to a carbon rich atmosphere and letting the carbon diffuse into the surface making the surface carbon rich and hard.
From all the data I have read, pre WW2 Mausers were made of plain carbon steels just like all of the other military firearms of the era. Plain carbon steels are inferior in all properties to alloy steels but they were state of the art in 1890, manufacturers are slow to change and legacy systems tended to use the same steels that they always had. The steels used in your Mauser would have been heat treated to withstand a 45,000 psia 8mm load. You could heat treat the material to a harder material, thus raising ultimate and yeild properties, but the material would be less tough. Toughness is a much more desired property in an environment where the load is basically a shock or impact load. The material would be heat treated just to the level needed to support the load and not more, as toughness would decrease.
Plain carbon steels were commonly used on parts prior to WW2, but metullurgy in the 1920's and 30's advanced so quickly that by the time you get to WW2 it is obvious that plain carbon steels are only a good choice if cost is the number one criteria and the loads are not high or safety critical. The steels used in pre WW2 rifles are now used as rebar, rail road ties, applications where the low load justifies the low cost.
The American metallurgist Edgar Bain,
http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/bain-edgar-c.pdf in 1932 published conclusive experiments on carbons steels. Bain heat treated identical plain carbon steel coupons under identical conditions and examined the coupons afterwards for hardness depth. The black chemical etching, which I assume is the unhardened steel, show that plain carbon steels have erratic hardening depths, given that all else is equal. These steels were called in WW2 era text books as “shallow hardening”. This was meant not as praise but as a pejorative. As is shown on the right of the diagram, the hardness of these coupons varies by depth. This is not good as consistent hardening provides consistent material properties. It is undesirable to create parts some of which will be hard through and through but others soft below the surface even though the heating processes are the same for all parts. But use plain carbon steels, and you will create such inconsistent parts, just by the nature of the material.
Therefore, you would expect even properly forged, properly heat treated plain carbon steel parts to vary considerable in hardness depth, which then affects the properties of the end part.
Yield is an extremely important material property, for above yield, the part deforms. Once a steel part yields it is no longer safe to use. What happens after yield is unpredictable, often it takes less load to cause more deformation, ultimate load is the load it takes to break the part. In this early 1920’s chart, for the same essential heat treatment, the nickel alloy steel always has a higher yield, a significantly higher yield in all cases, than the plain carbon steel.
Nickel steel versus plain carbon steel
What is not shown in these charts is a material property called toughness. For a device, such as a receiver, which is going to be subjected to impact loading, toughness is a highly desirable property. Toughness is directly related to fatigue lifetime, which is the number of loading cycles to failure. Assuming the yield is sufficient for the load, the tougher material will have a longer service life. Alloy steels have a greater toughness than plain carbon steels. Alloy steels take more energy to shear, Charpy impact tests are a direct predictor of a steel’s fatigue lifetime. It is a revelation to see just how shear energy decreases with temperature, and at low temperature, alloy steels take several times the energy to shear as do plain carbon steels.
Therefore, old plain carbon receivers are a very significant unknown quantity. We know that the material varies considerably in properties after heat treatment, and that the service life of the part will always be less to one made out of a good alloy steel. Just how many service lives have these old receivers been though? How many more load cycles will they take before failure? How will they react in an overpressure situation?
The fact that you have receiver set back in your rifle is of a great concern to me. The material has yielded indicating that an excessive load or loads have been fired in that rifle. Once a material yields, it takes less load to make it yield again (typically). It is your choice, but if you shoot the thing you better check the headspace to see if it is growing, and regardless, understand you are behind something that is less strong than it was when it was new. If that receiver had gone through a rebuild program, it would have been discarded if lug set back had been found.