> Although the MM-61 was originally intended for use in combination with dictation machines and the first generation of (large) hearing aids, it soon became a prime candidate for covert listening devices and recorders. When carried casually in the outer pocket of a coat, the microphone could barely be distinguised from a real fountain pen and would not raise any suspicion. Because it was overtly carried, the sound quality was superior compared to that of a concealed microphone
https://www.cryptomuseum.com/covert/mic/mm61/index.htm
> Although the MM-61 was originally intended for use in combination with dictation machines and the first generation of (large) hearing aids, it soon became a prime candidate for covert listening devices and recorders. When carried casually in the outer pocket of a coat, the microphone could barely be distinguised from a real fountain pen and would not raise any suspicion. Because it was overtly carried, the sound quality was superior compared to that of a concealed microphone
Also, the woodworm: https://www.cryptomuseum.com/covert/bugs/holzwurm/index.htm