Note
There were several variations through about 1946.
See National's notes: QST, Aug 1934, p. 69. (hints at new receiver)
QST, Oct 1934, p. 72, inside back cover (first ad)
See National's notes: QST, Oct 1934, p. 61.
See National's notes: QST, Dec 1934, p. 69.
See National's notes: QST, Mar 1935, p. 77.
See National's notes: QST, Apr 1935, p. 75.
Also see QST, May 1935, pp. 21-28.
See National's notes: QST, Jun 1935, p. 61.
See National's notes: QST, Mar 1937, p. 63.
See National's notes: QST, Mar 1938, p. 67.
A few notes in QST, Sep 1941, p. 67.
https://www.radiomuseum.org/forum/national_national_hro_the_evolution_of_the_national_hro.html
Micrometer Dial
The Micrometer Dial reads direct to one part in 500. Division lines are approximately 1/4" apart. The dial revolves ten times in covering the tuning range, and the numbers visible through the small windows change every revolution to give consecutive numbering by tens from 0 to 500, and provides about 12 linear feet of tuning. The condenser is of extremely rigid construction, with four bearings on the rotor shaft. The drive, at the midpoint of the rotor, is through an enclosed preloaded worm gear with 20 to 1 ratio. Each rotor is individually insulated from the frame, and each has its own individual rotor contact. Stator insulation is steatite. Plate shape is straight-line frequency when the frequency range is 2:1.
References
Review. Radio News, Sep 1935, p. 150.
Review. Radio News, Oct 1935, p. 208.
Discussion. Radio News, Jan 1936, p. 398.
