I am having trouble believing you can't measure the threads, and once you do,
sourcing a proper nut. A grinder arbor nut is made quite differently from some
hardware store item.

Measure the OD of the threaded shaft with calipers, as accurately as you can,
and use a thread gage to get the number of teeth per inch. If you don't have a
thread gage, then hold up a good ruler and count the number of teeth in a
quarter inch and multiply by 4, or count the number in a half inch and multiply
by 2 if the threads are long enough.

Post what you come up with.

Many T&C grinders use wheel holders like this one:
http://www1.mscdirect.com/CGI/NNSRIT...PMT4NO=7825604

All such grinders have very similar dimensions on the business end of the
grinding spindle. Therefore it is very likely indeed that a nut from one may fit
yours.

I also suggest you find a way to go look closely at a unit like the one you
bought. There are machine shops near most people in this country, and a lot of
them probably have a T&C grinder.

GWE