Hello Tooly and TM51,
I don't know what the threads were on the stock vise.
I have made my stop universal (for my shop) in that I fit it on several vises.

My stop rod is threaded 1/4-20 on one end and 5/16-24 on the other.
This arrangement fits all my vises.

I believe that I simply drilled out the original threads with a .201" drill bit
and then threaded it 1/4-20.

Anyhow that is how mine is setup.

Cheers, JR