Hi Tony,
Can you get to both ends of the headstock shaft to measure? You could try measuring the diameter of the headstock taper (at least at it's mouth) then compare / check using a table from an internet search:
https://en.wikipedia.org/wiki/Machine_taperOr, if you can get to both ends, does the headstock definitely have a taper? Perhaps there's been a bit of matching / reaming been done and now its a straight shank? A threaded bar and a locking nut might have been a modification to hold a fitment in the headstock securely (as an alternative to a taper fit).
Hope this helps,
