Thanks Harvey, Raymond, Craig, HenkC and Roger for your last contributions (To all others as well, I can't catch up with all the answers).
I must say that it's a joy to participate in this group. So much knowledge in so many areas!
I will summarize my comments... To avoid too deep threads.
On Raymond's and Craig's clarification to Harvey remark on the direct interface plug-in (and required modification of the MF)... Wow, connect directly to the CRT's plates...that's a radical approach! I imagine the equalization networks required to "tame" the interconnect cables characteristics, as well as to compensate for the plate's capacitance...
On HenkC's remark about the requirement (or lack thereof) for a standardizer... Acknowledged!
As this thread progressed, I was able to gather a few pros and cons about following the "strict" path to calibration of this unit, versus a more flexible one...
My conclusion was that, at least DC wise, which is relatively easy to achieve even without a standardizer or an extender, just a DMM, I will pursue getting the MF calibrated to standard (thanks Roger, for the suggestion to tap into the Plug-in Output's and for the reminder on the "isolated" DMM... Never fear... I don't have a bench DMM, only a handheld one).
As for AC and Step response wise.. I think I will just try to get the best system wide response of the set (7623A + 7A26) calibrating the whole thing as if it would be one.
On Roger's answer... Yes you were able to add. Indeed it was the first time I saw an reference on what is the actual expected deflection factor (50mA into 50R / div).
I sort of knew that it was around that ballpark, by looking at the 7B53A output waveforms, and wild guessing that the deflection factors adopted for both the horizontal and vertical compartments were the same (it had to be, if X-Y operation was to be accurate by any measure).