National manufactured a number of popular receivers during the existence of the company. Two of these receivers stand out in my mind because of my history with them, the HRO-60 and the NC-303. Both of these receivers had multi-band coverage (beyond HF -- I know the NC-303 required converters  but the band spread was there) and represented significant technology in communications. These products brought it all together on their introduction..

The HRO-60
The HRO-60 covered 50 kHz. through 54 MHz. in near general coverage with some overlap when changing coil sets. Plastic strips are changed when coils are changed to track the frequency band spread (see the Coils page). The HRO-60T (T = table mount) had several accessories in addition to the coils. The speaker HRO-TS, NBMF adapter NFM-83-50, Crystal Calibrator XCU-50-2, six volt vibrator power supply 650S and the Select-O-Ject SOJ-3. The SOJ-3 (I have never seen one) acts as a notch filter.
 

The NC-303
The NC-303 covered the Amateur Service frequencies from 1.8MHZ. to 225 MHz. using converters on the VHF/UHF bands. The converters use the so called "X"  band 30-35 MHZ. as the first IF. The rotating band spread drum covered all of the frequencies.  The model XCO-303 designated the crystal calibrator. This unit  included the crystal for 10 MHz WWV reception.