[CarbonMike]

I have been looking through my old QSTs from the '30s and '40s and the HRO design seems very interesting.
When it was released in 1935 it already looked out of date. But it was manufactured for another 10 years in more or less the same form, and it was copied in several other countries. Why?
The plug-in coils were the main point of difference. By 1935, other radios were appearing with bandswitching and direct frequency readout, which you might imagine would kill off an HRO-type design.
Evidently National had plenty of customers who didn't mind plugging in coils and reading little graphs. A lot of the commercial customers probably used only one or two frequencies anyway. Radio amateurs had all the time in the world, of course.
Having coils which were plugged in 'sideways' was a stunning innovation. it meant you had space to mount those little graphs, and you could reduce the overall depth of the radio. The tradeoff was the need for an east-west main tuning capacitor, and therefore for a rather complex gearbox and dial. National did such a good job of this that they sold them to the HRO clone-makers as a standalone component. Even today these capacitor/gearbox/dial assemblies turn up at junk sales in New Zealand, 'new old stock'. They were probably left over from production of clones by local manufacturer Collier and Beale. Now they change hands for a few dollars.
Also absent in the HRO was the PSU and loudspeaker. The cabinet was small but it was nice and cool in there. As we now know, lots of HROs were used in
WW2 monitoring stations, dozens at a time, so they could use common power supplies.
Before the Collins TCS series came along, they were also standard kit in small naval craft like LSTs. This would be a severe test for any radio. My guess is that the sheer simplicity of the HRO, and its build quality, made it a very reliable and serviceable item, and thus favoured by the military.
Note that the HRO was never a cheap radio. In 1940 only the Hammarlund Super Pro was more expensive.