Lot 3 industrial RFL FS receiver/discriminator HB-21045-51

Lot of three industrial telecommunication radio receiver modules. These are marked "FS Receiver", model number "HB-21045-51" and "30KHz". They are marked on the backs "Discriminator", "RFL Industries - Boonton, NJ - HB-58800" and "30KHz - FS +- 500Hz". 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.