I came across the following from tymkrs where they discussed harmonica microphones. There seemed to be some somewhat contradictory info on google, so I asked them for some clarification. In a matter of 20 minutes, they sent me a link on the internals which filled in a lot of the back story.
A little bit of further digging, and I found out a bit about the fabrication of rochelle salts. Its a fairly old school albiet simple method of crystal growing… and it got me thinking a bit more. Ie, how are the lead wires attached, and how is the rochelle salt protected from damage.
A little bit on google patent search provided the Astatic Corps patent. Like many things of that era, labor was inexpensive, so the emphasis was more so on consistency than labor saving. In addition, the variety of materials to use in manufacturing was pretty limited.
Ultimately, this little bit of digging tonight opened up a huge number of ideas, not only in the microphone domain, but also for percussive sensing including instrument keys.