Interesting one this, Bruce,
in one of my books circa 1929, there are exotic threads described in great detail such as, French Standard System, USS Small and Whitworth Small etc., but the only one to have 30tpi, is The American Society of Mechanical Engineers, ( 60 degrees ), Number 10, with a nominal diameter of .190, tapping drill 24. I don't know if this is of any use, probably not, but it is interesting anyway.
go carefully mr hutch
The screw that holds the cam on the shaft of a '28 - '31 Ford distributor is .325-30tpi. On the forum where I read this, the members, who presumably have older Machinery's Handbooks, are unfamiliar with that thread. Do any UK fellows here know of that thread?