Radio Beam Filter NAF 68304

The NAF 68304 Beam Filter is a passive audio device connected in-line with headphones from a US Navy aircraft navigation receiver.  It was designed to filter the audio frequencies to peak 1020 Hz, the modulated tone used for the A-N aircraft navigation Beam frequency.  In the A-N system, a steady tone meant equal audio from both the "A" and the "N" Morse transmissions which meant the aircraft was flying "on the beam" directly toward the selected navigation transmitter.  Hearing either the "A" or the "N" more distinctly meant the aircraft was flying to the right or left of the navigation beam.  A beam filter would limit extraneous noise or nearby signals. The switch to phone allowed full audio or selecting both still emphasized the range audio.  Note that it can be mounted in either vertical position.

The phone plug and cord are permanently attached.  NAF stands for Naval Aircraft Factory.  Note the orange anchor stamp indicating US Navy acceptance.

In questo link sono elencati 407 modelli, di cui 359 con immagini e 213 con schemi.
Elenco delle radio e altri apparecchi della MILITARY U.S. (different makers for same model)