Had an S23 back when I lived in Florida. That was in the 80s. Too bad they don't make them anymore, they were good boats. It was perfect for weekends on Tampa Bay and down in the Keys.
Anyway, as for the pin, it is just a stainless pin. Measure the size you need and I would be absolutely SHOCKED if you can't get exactly what you need at mcmaster dot com.
When I had mine, I also put a quick release lever on the backstay. Go to mauriprosailing dot com and search for "quick release lever." You'll see what I mean. Between that and quick release pins everywhere appropriate it made raising and lowering the mast a snap, because you could release the tension while raising the mast (making it easy to get everything hooked up) and then instantly reset the tension.
Thanks for the link to mcmaster, I've found my new favorite site for bits of hardware.