Among the tuning units available for this set is a curious example shown below for the transmitter that covers the 4,000-5,000kHz range, but can rely on a crystal to establish a specific operating frequency within that range. If the crystal is plugged in, the frequency control on the transmitter doesn't affect operating frequency any more,so the full range of the tuning unit is limited to the crystal frequency of choice.
Unlike the other coil sets of the time, this one was apparently not manufactured exclusively by ARC or Western Electric. The crystal itself has an unusual DC-10 case that is retained in the circuit by two tiny wingnuts. The frequency in most of the samples I have seen is 4,495kHz, a tower guard frequency for USAAC and USAAF aircraft in the late 1930s and up through the war. Its issuance suggests it may have been created primarily for training aircraft, to avoid giving the student yet another thing to keep track of and potentially cause a dangerous situation.
Compliments of Hue Miller, below is a seven page set of instructions for use of this unusual coil set.