Bob wrote:
> 
> I don't have a thread gauge at hand; so I can't determine the number of
threads per inch.

This sounds like you have a clear need for another tool, namely the thread gage.
Clearly, this is a moment that a galoot should relish!

Consider that if it is in fact an oddball thread then finding matching nuts may
be challenging!!!

I’d try-fit nuts of the standard coarse and fine threads for that diameter.  If
you find a fit, that is your de facto thread gage!

Proceed carefully!  That rod may be “staked” into the base with a deliberately-
distorted thread so that it is a jam-fit.  Forcing this to unscrew may leave it
as a too-loose fit.

Use penetrating oil a few days before you apply torque to that rod.  (I prefer
Kroil from www.kanolabs.com)

It might be best to simply not remove that rod.  If all the corrosion & crud are
removed from the exposed part of the rod, then removing the rod from the casting
might be a case of “don’t fix it if it ain’t busted.”

YMMV!
John Ruth