Lot of 3 industrial radio RFL BF oscillator / demodulator HB-25640-2

Lot of four industrial telecommunication radio modules made by Radio Frequency Laboratory. These are marked "BF OSC-DEMOD", model number "HB-25640-2" and " 28.3KHz". They are also marked on the backs "HB-63300", "HC-25642" and "BW 500 Hz". They measure about 7" x 3 1/2" x 1 1/2".

There are actually four of these radio modulators in this lot, but one missed being photographed.

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.