I did a bit more reading of Steven Matthews manual and exchanged emails with him.
Again, SM says its a guide and it means just that. He also is clear its for the hobbyist and thats both honest and I appreciate how clear he is about it all.

The SM (former Axis engineering) gauge per Steve is not intended for or calibrated for the 1917. I did request he look into it as it does appear to be close of if not valid.

In our specific case, the Eddystone I have, actually measure a MW of 3/4 (I had missed that the first mark was zero). The other two 1917s measure 2. (MW). I would say that validates its usefulness as a Guide to MW.

The TE on the Eddystone measures 1. That corresponds with the MW and the gun appears to have been shot very little and it never underwent went any Arsenal work. The stock screws are still in the original staked position (you can't break them loose) and the gun has the original dark black finish (and not reimporting finish). Its an excellent baseline.

The other two were re-arsenaled, probably post WWI from the appearance of the parkerization and subsequent wear. Both either had some use or the TE and MW due to mfg difference are slightly different (barrels are original as near as I can tell as dates are withing a month of receiver mfg) Both are 2 on the MW and 2- to 3 TE.

So, while Steve Matthews is right to say what he does because he has not either calibrated it or done a specif 1917 gauge, I am more than confidant that it works very close to if not absolutely correctly for the intent (Guide) and thats to give you a good idea what the TE and MW are on the 1917s.

Hopefully with the popularity of the 1917 on the increase he will offer up the calibration offset value (if any is really needed), but I am convinced that its good enough for what we need. I am inclined to a calibration (or confirm its so close as not to need one) as that would the Gauge perfect to use on the 1903s and 1917s.