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National's "Moving Coil" receivers utilized a cast-metal catacomb that carried the various front-end coils and placed them into contact with the tuning condenser by way of small contact pins and pin-receiving contacts.
The catacomb was rack and pinion driven with a large knob on the front of the receiver.
It's possible that National designated the HRO-5A1 receivers built after July, 1946 as the "HRO-6" for a short time.
The matching speaker was also restyled to match the new HRO-7's appearance.
Internally, a voltage regulator was added and the LO changed to a 6C4 miniature tube.
Coil sets were changed to not employ handles for removal and levers were utilized.
It appears identical to the HRO-5A1 externally and the only difference between the two receivers is a slight change to the Noise Limiter circuit.
It's likely that the actual change in the Noise Limiter is that it was built directly onto the chassis instead of being built on a small chassis that was mounted on top of the receiver chassis.