I have just started to assemble a Roundhouse 'Lady Anne' loco kit.
The RC fittings kit includes the steam regulator mentioned by Nick, so I have taken some photos of it. See my 'RC steam regulator' album.
Not sure what the threads are but no doubt Roundhouse will tell you if it looks suitable.
Stuart
Edited By Stuart Smith 5 on 22/11/2018 12:12:55