The fixed rear element in the capacitor/diaphragm of a condenser (capacitor) microphone.
"What I sent you ws a high fidelity record I first heard in 1957. It has been played all these years and moved 19 times, four times across the pacific ocean. It is very dear to me and I was afraid to play it anymore (even if I still had the equipment which I now do not) for fear of losing it forever. When I received your package and put it on ..."