Kenner Proton Pack gun help!

I just repaired the trigger on my Kenner thrower, but when I popped it open all the gears flew out. I got everything back into place except for this small metal disc with the four holes in it. Does anyone know where it goes? I’m sure it has to do with the sound but I can’t mount it anywhere.

Nevermind I found a picture of it; it is mounted alongside the smallest gear.

Side note, has anyone ever considered doing a small run for the tripper/gear assemblies? Two guns have broken in the same spot on me.