[Arran]

Given the quad set of power output tubes that many of the big Midwests seem to have they likely needed the pair of 5Y3s to power it all, though perhaps they could have got by with a 5U4. The separate mixer and oscillator tubes was actually a useful feature for shortwave listening, at higher frequencies pentagrid converter tubes tend to get electrically noisy. The dual diode second detector was a common act of tube stuffing in those days, usually paired up with a 6J5, many RCA, G.E, and Canadian Marconi sets did the same. I've also see this trick used in some Strombergs, and in one Northern Electric I serviced, again a useless feature.
In an 8 tube Canadian Sparton set I have, I think a Model 89 from 1938, it uses a 6H6G, an ST style 6H6, something I have never seen before in a radio, most are stubby metal tubes. However both diodes are wired together as a second detector, they didn't even use the second diode for AVC purposes. They used two 6J5s and two 6F6s in the audio output stage, one as first audio, the other as phase inverter, then as was common, they also had a Magic Eye, so in effect its a very loud five tube set.
They could have used a 6Q7 to do the same job in most cases, Rogers sets often used a 6R7M (6R7), like they did in my DeForest Crosley Brahms model. But in the next chassis up the line, the 10 tuber, Rogers used a 75M (6J5) and an 84M (6H6), but they don't do anything that the 6R7 doesn't do. This was obviously a marketing ploy since all of the circuitry they are connected to is identical, right down to resistor values. In case you are wondering about the "M" that means it's a glass tube with a metal spray shield, usually it's just a clone of a regular ST or metal tube sometimes with a different number just to make it sound special.
But as with many Rogers built sets from the 1937-40 period it uses two rectifier tubes, like a 2X3 or a 2Y3, with a single diode in each. The 2X3 is exactly 1/2 of a 5Y3, while the 2Y3 is 1/2 of a 5U4. This may have been for tube stuffing or some engineer's idea of a safety feature since the filaments were wired in series to add up to 5 volts. It's strange because Rogers used type 80s before they came out with these, and used type 80s after they phased them out.
Regards
Arran