Well, Thunder Ridge's web site suggests a Treso no. 11-50-30 is the one to get it's for a ".31 Revolver nip. 12x28". There is no cross reference to a TRM part number.
I have a Uberti 1862 Colt Pocket Police in .36 cal that I'm having some difficulty finding replacement nipples for as well. I believe it takes a 12x28 thread, but the Treso 11-50-16, which is a 12x28, won't start in the hole. I'm going to try the 11-50-30 next.