This is for a pair of AK40's 1 works and sounds great, the other rattles, the inner nut underneath the capsule is loose and I do not have the tools for it. I do not know if the second one works or not as I did not want to try to run power to the capsule.
If you know someone with the tools, I'm sure capsule #2 could work for you and for a steal.
AK... active capsules are part of the former Neumann KM 100 miniature microphone system. Together with the KM 100 output stage AK... active capsules form a complete microphone. AK 30 and KM 100, for example, make up the KM 130.
AK 40 is a pressure gradient transducer with cardioid characteristic. The frequency curves are very even and parallel to 0° sound incidence. Sound sources within a pickup angle of ± 135° are transmitted without coloration.