The seller says that it's #50 taper. I first guessed that this was only
the horizontal taper, since that seems to be the usual taper for
horizontal mills, but have since gone looking (and read the replys here)
and found that many versions of Cincinnati H/V do use a 50 taper in the
vertical head.

So I went looking (in MSCs catalog) for "things that fit a 50 taper" and
found many, many different prefixes on a 50 taper. CAT V 50, NMTB 50, BT
50, and possibly an ISO 50. My education in machining was primarily as a
Lab Tech, using the tooling that was associated with the tools we had,
so I'm damned if I know what the differences between all these 50 tapers
are. The argument that they are all the same seems questionable since
various things were offered in two or more of these prefixes.

I can see that buying any 50 taper toolholders at retail will quickly
exceed the price of the mill, and it's not coming with much tooling (the
disadvantage of working job shops .vs. dead people as tool suppliers -
the dead people's relatives often want "all that stuff" out of the
basement as a throw-in).

--
Cats, Coffee, Chocolate...vices to live by