Case hardening a low carbon steel & heat treating it leaves a hard Case (outer surface) over a softer core. Heat treating a higher carbon alloy steel gives much more through hardening. Case hardened steels are normally left with the case at or very near to maximum hardness, as the core is still soft. Heat treated alloy steels will normally be drawn (tempered) to the desired degree of hardness dependant upon their use. Case hardening is not the proper treatment for steel having above about 20 points of carbon (this includes the M21 frame). I am not a great fan of the M21, so not trying to eulogise it, but the blued frame on the 21 was not for cost cutting, it was because of the type of alloy steel used it was not the proper finish.
