just checked the thread on the Mamod safety valve. Admittedly, the diameter is 1/4 inch, but, I checked it with a thread gauge and the threads are 26 tpi.
When I have checked on the web - cos to me it is an oddball thread - it works out to be a cycle thread used by the cyclists industry.
I know the thread you are looking at is 3/8 diameter but, this Mamod being an 'old' English Company - I am wondering if it is 3/8 cycle thread. The other thing which came to me the thread on the 1/4 diameter was very close to the pitch and size of the O/BA thread.
That has just thrown a cat among the pigeons
I know there is a guy on EBay who I have dealt with in the past does a lot of spares for the Mamod steam engines - because he supplied me with a washer for my Meccano steam engine.
John