I suppose it depends where they source their fittings from, if mainland Europe then the various metric fine 0.5, 0.75 and 1.0 pitches are the usual ones used on steam fittings.
If English made then 26tpi would be likely as Colin says, A quick look at their catalogue shows their nuts for 3/8" pipe are threaded 9/16 x 26 so that would be the best bet.
Edited By JasonB on 29/06/2019 06:58:26