The acronym stands for "Super Regenerative Receiver" and I think it was their first foray into the old amateur radio 5 meter band, before the sophisticated 1-10 "Ultra High frequency" receiver was much more than a gleam in James Millen's eye.
It's a tiny thing, 7¾" x 7¾" x 7" and 5 meters is where it tops out; with a minor reshuffling of coil connections, it covers the lower ham bands, from 10 meters down to 160 meters.
As a "rushbox" super-regen without an RF amplifier to isolate the detector from the antenna, it's not a good citizen on any of them; in operation, it radiates a broad swath of noise, inherent in the superregeneration process.*
And I own one. There it was, at the Indiana Historical Radio Society's Spring swapmeet, standing out on a seller's table to anyone with an eye for old National equipment, at a remarkably low price. I made an offer and somehow ended up with a nice homebrew absorption wavemeter in the bargain.
Have a look inside the receiver:
The coil and socket are standard 4-pin "tube base" configuration, unusual for a National product; they usually preferred their own designs. But that will make it simpler to wind new coils. The one in the socket probably isn't for this receiver -- not just the "Hammarlund" name on it, it lacks the cathode tap found on all SRR coils. I may have coil-winding data. I'm certainly going to give it a try!
Rough schematic:
It uses a version of National's "S-101" interstage coupler that I had not seen before. The S-101 is a weak point in the design, as the internal audio choke (inductor) has a very high impedance, meaning many turns of small-gauge wire, which tends to perish in the potting compound. Checking that will be the first order of business.
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* I believe it was Howard Hughes' 1938 around-the-world flight that found radio communication nearly wiped out by interference from people trying to listen in with simple superregenerative receivers. This is why the later 1-10 had an RF amp, to keep that detector from getting in the way!
