For the needs of American flight communications, James Millen, an engineer of the National Corp. had developed an all wave receiver which completely changed the design of all commercial shortwave receivers - the earliest HRO appeared in 1934 and the set has been produced in different variations and with several improvements until the End of WWII.
The revolutionary concept of the high precision tuning condensator PW-4 in conjunction with the "Micrometer dial", a mechanical dial reading from 0 - 500 acting as logging dial, made retuning a station on a known frequency possible, the numbers could be written down in a logbook for later reuse. There have been calibration charts for the receivers, the numbers displayed on the dial of the HRO do not correspond directly with frequencies in kHz.
Bandswitching in the HRO is realised with coil sets, the four main shortwave coil sets can be switched from general coverage to bandspread coverage of the amateur radio frequency bands. There have been five optional coil sets to cover VLF and mediumwave frequencies.

The HRO was one of the earliest receivers featuring not only separate RF and AF gain controls, an automatic gain control (AVC) but also a signal strength meter to determine the signal level of a station and a crystal filter with a control to set the passband or rejection curve.
A separate power supply has been used, which made less problems with noise from the power transformer and heat dissipation: the frequency coils sets located below the main tuning condensor are less exposed to heat from the power transformer and the rectifiers which added to frequency stability of the receiver.
Over the years, National's HRO was delivered with different valve layouts included a solution using battery valves.

When you look at the outside of the radio, you can clearly recognize it's roots from commercially used receiving equipment. The metal cabinet comes with a hinged lid to access the chassis for changing valves from the top. An important accessory is the mains power supply, usually the "doghouse" shaped National 697 power supply delivering heaters and B+ / plate voltages. A wooden chest is used to store the coil sets not in use, the set could drive headphones or an external speaker.

The frontpanel of this very early sample of commercial receiving equipment is not that clearly structured as found in later receivers: most controls in the earliest sets came without lettering, but the operator simply knew which knob is for which control: The main tuning knob with the "micrometer" dial, on which the numbers of the logging dial can be read, is found in the middle of the front panel. On the coil set just below the tuning knob, there is a calibration chart which helps you determining the frequency of a signal. Some coils sets come with double frequency charts, one for general coverage use and the second one for bandspread use in the amateur bands. In the right column of controls, the top button, in early sets a bar type knob with an arrow pointer, lets you regulate the passband width of the crystal filter, the rotary control below turns the crystal filter on and off and acts als rejection control like a phasing filter. The next switch will turn on B+, the plate voltage: let the B+ switched off when the receiver is in standby, and B+ must be turned off when changing coil sets!
The bottom control at the right is the RF gain control.

At the left, you find the signal strength meter, in early HROs, it's activated by a pushbutton, in later variants by a toggle switch. The next control is the AF gain control, the volume control. The switch underneath will turn off the automatic gain control (AVC) when in it's right position, the bottom control activates the BFO for the reception of telegraphy signals.

Technically, the HRO is a single conversion superheterodyne with two RF preamplifier stages. The signal coming from the antenna will pass a first tuned circuit of the RF preamplifier (6D6, in the late version HRO-5 6K7) and a second tuned circuit of the RF amplifier stage (6D6, later 6K7). When mixed with the oscillator (6C6, later 6J7) signal in the mixer stage (6C6, later 6J7), the intermediate frequency of 456 kHz is generated. This concept using tuned RF circuits could be realised by National because of their excellent quadruple tuning condensor and the plug in coil sets.
The intermediate frequency will pass the crystal or Lamb filter where the bandwidth can be regulated and unwanted interfering signals can be rejected using the rejection tune feature. After two IF amplifier stages (6D6, later 6K7; and another 6D6 resp. 6K7), the signal is demodulated in the combined detector / AF preamplifier stage (6B7, later 6SQ7). For reception of CW morse codes transmissions, the signal of the beat frequency oscillator BFO (6C6 resp. 6J7) is injected at this point. The AF level is sufficient for headphones use, with the following AF final stage (42, later 6V6GT) a loudspeaker can be driven.
In the external power supply 697, you find an 80 rectifier tube for the B+ / plate voltage of 240 V, this and the 6,3 V heaters voltage will be fed to the radio by means of a four pole power cable.

In a list of receivers which were used in the Swiss Army, there are several types of HRO receivers. I suppose, that in the earliest time, maybe even single receivers got their own E-xxx designation. As far as i know, the HRO has never been acquired in large numbers, maybe, we can add more informations about the use of the HRO receivers in Switzerland later.
In a early list, I found:

• E-606: HRO No. 1 (later, the number E-606 has been assigned to a troop information / entertainment portable transistor radio
• E-607: HRO No. 2
• E-610: HRO Standard