On Thu, 18 Jun 2020 18:11:47 -0400
Leon Fisk wrote:

snip
Yeah, know exactly what you're talking about and probably have the one
you need. Think I have two of them, most likely fit old Shimano
5/6 sprocket free wheels.

Actually have four of them but only one splined. Checked an old Shimano
Freewheel that I know it fits and a 9/16 socket will slip inside. A
sharp edged 3/4 nut will catch the splines (12) in the freewheel...
Sounds like you need one a bit smaller in diameter than this

With all the metric and standard nuts around nowadays I would try
modifying one of those first. Even better if you happen to find a
coupling nut that fit, would give a longer length to work with...

--
Leon Fisk
Grand Rapids MI