sets of coils, E and F, sold separately, covered 960–2050 kHz and 480–960 kHz, respectively. Before each radio left the factory, a technician custom calibrated a set of A, B, C, and D coils for that particular radio, a process that took nearly 4 hours. Each of the four main sets of coils also had bandspread modes set by moving screws that limited the frequency range to 28–29.7, 14-14.4, 7-7.3, 3.5-4 MHz, respectively, for amateur radio use. Models Main HRO models: HRO (also called HRO-Sr, 1935–1943) HRO-Jr (February 1936 – 1943, US$100 version of Sr with only one coil not individually aligned to the receiver and lacking crystal filter, phasing control, and signal strength meter) RAS (1939-?, HRO-Jr for US Navy with general coverage coils and 175 kHz IF so the radio could receive the 500 kHz distress frequency) HRO-M (used in conjunction with Bletchley Park) HRO-5 (1944–1945, octal tube version) There were also several sub-variations on these models. Production The US military told National, "Start building HROs. We'll tell you when to stop." Before, during, and after World War II, the HRO concept of using plug-in coils with micrometer tuning was copied in several countries, including Germany and Japan. The best known copies are probably two German models widely employed as monitoring receivers by the German services, the KST made by Korting Radio and the R4 made by Siemens. Following World War II came the HRO-7 (1947–1949, 12 tubes, including 2 miniature tubes), HRO-50 (1949–1950, built-in tuning dials and power supply, push-pull audio amplifier, improved styling and performance), HRO-50-1 (1951, increased IF selectivity), and HRO-60 (1952–1964, dual conversion for coils B (7-14.4 MHz) and A (14-30 MHz), heater current regulation for the HF oscillator and mixer tubes). These were followed by two solid-state receivers that did not use plug-in coils: the HRO-500 (October 1964 – 1972, 5 kHz - 30 MHz, and HRO-600 (1970-1972?, 16 kHz - 30 MHz). Breaking with tradition, the HRO-600 used a frequency counter instead of a micrometer tuning dial. Ironically, the HRO-600 used nixie tubes for its digital frequency display and thus was, technically, no longer "all solid-state" as its predecessor, the HRO-500, had been. HRO receivers were said to be outstanding and continued to be popular although even better and more expensive general coverage receivers from such companies as Collins Radio became available in the 1950s and later. One can still find HRO