Lot of 3 industrial telecommunications RFL FS receiver filter HB-58700

Lot of three industrial telecommunication radio receiver modules made by Radio Frequency Laboratory. These are marked "FS Receiver Filter", model number "HB-58700" and "Center Freq 30KHz". They are also marked onn the backs "BW 1200 Hz". They measure about 7 1/2" x 3 1/2" x 1 1/2".

These are from the estate of an industrial telecommunications engineer and ham radio enthusiast. We don't know much about them and we have not tested them.