Worse than being Imperial which is fairly available they are a strange American thread called UNF
, Its a bit like Gas Gauge, an extra fine thread that is used in specialised work where they dont want the public joining stuff up to it. Sometimes you can get parts from specialised fastener or plumbing suppliers.
Corny Kegs are 0.594 NUF
Firestone are 9/16 UNF
There were adaptors available at one time to convert from Cornelius to BSP for use as washout kegs with line valve connectors think there might even be a couple in my miscellaneous parts box defiantly Corny tho.