Good point, Jon! I'm accustomed to being VERY careful when torch-heating pot metal, zinc alloy, etc. Another option would be to carefully drill the fitting out until you're close to the minor diameter of the thread, then chase the thread with a 1/8 NPT tap. If it's necessary to cut it slightly deeper, no problem- - - -the thread is tapered. I think I've got a couple of Rochester B paperweights, or parts of them, anyway, so if a replacement air horn is needed, I could probably provide one.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!