Quick followup. I tried the Brown and Sharpe #10 reamer that Jeff sent. It fits perfectly in the spindle. So at least I can confirm the mystery taper in this DH is a B&S #10. The small end of the B&S #11 fits also, but only part way in. Same taper/inch. Just a larger size...
So, for anyone running into this problem in the future, particularly with a VN 10 DH, in stead of trying to measure the taper and length of the tooling, try measuring the dims of the openings and length of the spindle. Then compute the taper per inch, or, taper per 6”. This might short circuit the process of chasing the arbor taper. It’s what I should have done in the beginning.
Also, my spindle end on the DH is slightly narrowed due to accumulated scale and dried oil from 85-90 years of sitting on a shelf in the machine shop- so will give a slightly incorrect taper. Mine was/is maybe 10-15 thou smaller ID due to the solid, smooth crud buildup over the years. Clean and polish it to bright metal before lifting the ID.
Now, to figure out where to buy some #10 B&S straight taper collets for work holding...
+ a big shout out to Jeff Anderson for loaning me his #10 and #11 B&S reamers! Without which I could not have resolved this!
Many thanks Jeff!!
Glenn