Specific Frequency Quartz Crystals
In the old days of radio, operators had to use fixed frequency crystals in order to transmit their signals.
The technology has gone digital and the circuts can not create any frequency for the operator, but old equipment still needs the crystals to function and the art is a dying one.
You can get crystals from a few places and here is one:
http://www.af4k.com/crystals.htm