I have come the conclusion that since the 5.5 x.9 threads are nearly identical to the 12-24 I will have to try the 5.5X.9 threads. If they don't fit then they don't fit. I measured the nipple with my trusty sawtooth thread gauge once again and it for sure is not 28 threads per inch. The 24 TPI gauge sits down into the tread perfectly the 28 TPI is not even close.
This gun is marked Replica Arms El Paso, TX so maybe the nipples came from over the border. LOL
Thanks for your assistance I really do appreciate it.