Klaxton help

[65rat]

I've picked up a Klaxton horn and the set screw on the diaphragm was stuck. We've drilled it out without messing up the threads but can't find a replacement for it. My question is does anyone know what size and pitch it is. A 12_28 is close but too coarse of thread
Thanks Tim

 

[horsefarmer]

I don't know for sure, but I think electrical switches, outlets ect. use a different finer thread?