Bryan,
My rod measures 19.9mm +/- 0.05mm measured with verniers - 3/4" is 19.05mm so it's 20mm rod.
I don't have any thread gauges or thread tables at home so I can't be definite about the thread other than to say it is not metric - 20 pitches measures 40 and a bit mm... 16mm metric coarse thread would be a neat 40mm. I would be 90% sure its a 5/8" UNC thread.
The keeper thread measures 5.7mmOD. A standard 1/4" camera mount thread (which I think is the same as 1/4" W
) does
not fit. I would expect it to be a 1/4" UNC but I can't be sure...
Al.