more on my AA S510

more playing with my new to me S510 . well another mystery ? went to try a moderator on this gun , simple just unscrew the thread protector and ??? what not 1/2 x 20 ?
looks to me it is a threaded barrel measured @ 37/64 diameter threads ?? Owner had a mod on it but did not sell with the gun . looking on the net and nothing found .
Donny FL has a male to male adaptor but states it will not work with a S510 that came with a moderator ?

If you bought a AA S 510 with a factory mod you can't change the adaptor ? and what is that thread anyway ?