As for the WHY of the mark, my guess has to do with breakage. Ruana was always known for, and even a motto of sorts, making a knife which would not let a guy down in the middle of nowhere, unlike most all of other designs, as there simply was nothing to shrink, rot, crack, or come loose to the design. Which led to some mighty harsh use among buyers.

I kinda guess the M stamp was in answer to such abuse, where, about inconceivable the M stamped stuff could bust under any sane use. Where L6 saw steel might hold a great edge but might chip on bone. Just a guess, mind you, but L6 blades might have been far easier to get replaced than the M stamped blades, where Rudy might have rightly said you were breaking an M stamp blade nearly on purpose.

All conjecture, unproven, unverified, which means it now will be repeated as fact over the entire internet for the next 20yrs, even showing up in a Ford antique tractor blog.

I knew about micro testing, but not the ultasonic. It causes me to wonder though, if we are entering same as with pressure testing, copper and lead crusher units, measured physical deformation against a scale, vs piezo strain gauge electronic method of psi or metric equiv...no direct correlation.

And since ultrasonic is obviously not attempting to dimple a sample under a known pressure, and then deformation measured against a scale, it will only be approximate to original method, and truly valid only in reference to other results obtained with same technology....just as with pressure measurments...only valid when relative to results obtained the same way.

The UCI method (Ultrasonic Contact Impedance or modified Vickers procedure) – first put to use in 1965 under the name “SonoDur” - electronically evaluates each Vickers hardness test indentation in a fraction of a second and displays the results digitally. As with traditional Vickers hardness testing, the material surface is exposed to the tip of an indenter with a defined geometry (= Vickers diamond) with a predetermined force.
However, unlike the traditional procedure, dynamic coupling impedance measurements are used to measure the hardness value under load. For this purpose, a mechanical resonator (vibrating rod) equipped with an indenter at its tip is excited to longitudinal vibrations of about 78kHz and forced into the test material. Through this coupling of materials, contact resistance arises beneath the Vickers diamond, thereby causing a dampening of vibration amplitude and a simultaneous increase in vibrational frequency in the characteristic manner. This frequency shift is exactly determined once a predetermined test force has been achieved and the Vickers hardness is derived immediately therefrom. The resonating frequency of the vibrating rod increases as the test force (or contact surface) has more of an impact. If the test force is constant, the deviation of the vibration frequency (= differential frequency Δf) from stand-by frequency f0 in the air is therefore an indirect measure of a material’s hardness. However, differential frequency is also dependent on the elastic property (e-module, Poisson's ratio) of the resonator, indenter and test materials. Since these parameters are generally not known, the system must be calibrated for probes of a known hardness (= normal, reference plates). It will then be easy to calculate the hardness from the differential frequency, test force and calibration data.