The American Airlines DME is another generation. The first generation had a CRT display which required interpretation to read the distance although later versions did have a meter for output.
As far as I can tell from these notes I have from aeradio staff the first DMEs, the ones dating from WWII operated on 176 Mhz whereas the Australian developed DMEs operated around 200 Mhz and the later DMEs 1000 Mhz.
The early DME were called Racon, for radar beacon and were in effect the reverse of the modern transponder.
In the case of the modern transponders the ground radar transmits first and the aircraft transponder responds and the ground radar interprets the response. In the case of Racon the aircraft transmits(ed), the ground station responds and the aircraft operator interpreted the time of response to determine the distance.
One of the aeradio operator described the equipment he was familiar with:
The original US Forces Racon was a very simple piece of equipment operating on 176mHz. A receiver on the ground, coupled to a free running oscillator biased to beyond cutoff, used as the transmitter, frequency controlled by a simple quarter wave tubular line. When a pulse was received, a blocking oscillator fired, the transmitting oscillator fired for a very brief period. The time off (gaps between pulses) was controlled by a simple C/R circuit.
Radar beacons were first used as early as 1941 and I suspect some are still in use in the marine world.