[Milsurplus] Mechanical filters with 128Hz spacing

[Ian Wilson]

I have a couple of mechanical filters, almost 6" from top to base, grey,
cylindrical, 7/8" diameter, 9-pin miniature tube base.

These have centre frequencies of 22.744kHz and 23.385kHz. I can't
measure the BW but it's under 1kHz.

There what look like channel numbers (120 and 124). If that's what they
are then the channel spacing comes out at 128Hz.

Anyone know what these would have been used for?

73, ian K3IMW