Unless I was told wrong by a master gunsmith, all the machine screws are Whitworth.I have never found an exception unless it was a replacement screw. An early set from CSMC were close,would thread in, but were not correct, do not know if they ever changed that in later screw sets?

Robert: I surfed a little and found this site. They seem to have any size one might want. I have not measured any of the Parker screws to determine diameter and tpi. If you already know that info, could you look at the list and note which are the sizes a person would need to be able to clean rusted threads or repair slightly damaged threads.
Thanks,
Jack

Sorry Jack, I don't know without checking, but it sounds like a worthwhile endeaver.
I'll have to break out my thread pitch gage and get to work. :)