My Yani T-990 neck is regular brass, not silver. The T-901 neck was also brass but did not have the under slung octave key. I have since sold the T-901. I think Selmer necks fit Yani, with some minor modifications, but getting back to King S-20…
I guess if you alter the neck diameter on your King or use some sort of shim in the Yani neck, in addition to getting the octave lever extended, you might get the Silver neck to fit better. The shapes of the necks are different and this may effect the intonation. It might not be worth the effort.
I decided the difference in sound that I would get from a Gloger Silver neck on my all brass S-20 was not significant enough to justify the expense. My S-20 is an exceptional horn as is.
Bethlehem has Musikfest in August. There are stages set up all over the place and most of the music is free. The festival lasts 10 days.