Wow, it is a magnificent thing! After years of struggling with small Jackson Brother reduction drives and with reduction drives brutally cannibalized out of innocent Heathkit Q multipliers, I now realize that I have been playing in the minor leagues. This, my friends, is the reduction drive that helped win WWII! I will have to build something worthy of its inclusion.
The designation HRO has a wonderful story behind it:
This is from: http://www.cryptomuseum.com/df/hro/
The new radio was also designed by James Millen at the National Radio Company, but this time with two RF amplifiers and two IF amplifiers at 455 kHz with a 20Hz crystal filter. He kept the pluggable coil packs as part of the design and added the now famous epicyclic dial, which allows the operator to tune the frequency scale in 1/500th units (with the aid of a calibration chart).
The design was finished in 1934 and National pushed hard to get the receiver out by the end of that year. When creating the tools for the first production run, the tool makers had to work overtime and used HOR (Hell Of a Rush) as a job number on their overtime slips. As National's marketing department didn't want their radios to become known as HORs (whores), the name was changed to HRO (Hell of a Rush Order). Despite the best engneering efforts, technical problems delayed the release of the the radio until March 1935. The price at the introduction was US$ 233.
Another site provides tech details and history on the drive itself:
The HRO dial introduced by the National Radio Company in late 1934 was the hallmark of top-of-the-line National receivers from the mid 30s through the 60s. By late 1936 the "HRO dial" was appearing on the NC-100 series of receivers and even the 1-10, National's VHF receiver. Throughout WWII many of the NC-100 variants that National provided to the military used this same dial. By 1950 National had added built-in direct frequency readout to the HRO-50 but still kept the same 0-500 reading dial. Through the mid-50s and into the 60s National mimiced the HRO dial look on their mid-priced receivers such as the NC300, 303 and 270. Even the solid state HRO-500 introduced in the early 60s used a version of this dial. When combined with the required 20 to 1 venier gear drive, the HRO dial provided an effective scale length of 12 feet and was direct reading to 1 part in 500. Ten turns of the dial drives the tuning capacitor stop to stop. Published HRO calibration curves showed each ham band spread over eight turns (or 400 divisions). In addition, dial divisions were about 1/4 inch apart. On all bands below 10 meters the HRO dial is easily resettable to within a KC (or KHz).