At least you know where your drawbar is.....
Thanks for the comment on the dimension in the comment. I was a chemical engineer before I retired, so my mechanical and drawing skills are still developing.
I looked at Colvin, 1945 and on page 396 he comments "Hole tolerances are all from basic to .001 inch oversize."
I looked in my drawer of cutters, and found I have very few 7/8" cutters. Most are gear cutters. And their keyways measures about .133 wide, not anywhere near .250.
I did find an interesting table on page 1360 in my grandfather's 1943 Machinery's Handbook "American Standard Keys and Keyways for Milling Cutters and Arbors".
For 7/8" arbors, they specify 1/8" nominal keys, with a range of .125 to .126 for the keyway. The bottom of the keyway is .8075 to .8125 from the opposite side of the shaft..
For 1" arbors, they specify 1/4" nominal keys, with a range of .250 to .251 for the keyway. The bottom of the keyway is .8438 to .8388 from the opposite side of the shaft.
For 1 1/4" arbors, they specify 5/16" nominal keys, with a range of .3125 to .3135 for the keyway. The bottom of the keyway is 1.063 to 1.058 from the opposite side of the shaft.
On page 1361, they have a table specifying the "American Standard" hole diameters for single angle cutters, exact diameter to .001 over. Page 1375 shows the same for Stagger tooth milling cutters and slitting saws.
So, the picture must be a 1" arbor with the .250 keyway. Measuring the flange and the arbor, it also works out to 1". I also noticed that the keyway just runs off the end of the shaft, as opposed to my drawing.
One other area that I really need confirmed is the length of the MT2 up to the flange, and the depth of the drawbar thread hole. New drawings in a few days.