They were referred to as a RADAR (RAdio Detection And Rangefinding) Range because they used a Magnetron for the heat source. As I recall Amana was the first one with them, under that name, and that was around mid to late 1960's.
Magnetrons are inexpensive (considering) devices that some lower technology RADAR systems use to propagate microwave energy out into space. It bounces off "targets" and those signals are returned to the receiver in the radar equipment and is displayed for the operator to see. It was developed during the second WW by British and US interests. The US involvement was at MIT. It was far superior to any radars available at the time.
The magnetron in a microwave does that too only the engergy is confined to duct work and the cooking chamber. The frequency of the cooking magnetron is 2.4 GHz as I recall and was selected because that is the frequency that resonates water molecules. Resonating them causes internal friction and they get hot. It was accidentally discovered by a physist at MIT also. He was conducting an experiment on something and noticed water bubbling in an adjacent beaker. So was born the magnetron to the kitchen.
The magnetrons used in microwave ovens are the most simplistic of magnetrons. Basically they are a microwave cavity with an output port powered by a high voltage transformer stepping up 115 volts. The power applied is sinusoidal (regular household power) but the magnetron only conducts when the cathode (electron emitting device) is negative to the other element....the anode. So it's self rectifying and saves the cost of adding those circuits. Raytheon, in Waltham, Mass. produced them for years in a very large assembly line. I saw it.
The grilles in the glass front have holes that are less than 1/4 wave length which prevents them from propagating microwave energy out into the room. The metallic walls and sharp angles in the door sealing area prevent leaks elsewhere, by design.
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