Originally designed in 1934 to meet a contract for a very stable aviation ground receiver, it derived its name in part from the penciled notes written on the various work orders for engineering and design. The note "HOR" stood for "helluva rush". When it came time to name the receiver, the initials were switched. So the origin of "HRO" can be interpreted as standing for "helluva rush order". The mechanical engineering was the work of James Millen and the electronic design was by Herbert Hoover, Jr., son of the former United States President. Millen mentions the development of the receiver in his letter-ad in the August 1934 QST (page 69). A two page description of the first HRO is found in the National catalog which was included in the October 1934 issue of QST. See pages 72 and 73. Millen apologized for the HRO not being available during November in his letter-ad in the December 1934 QST (page 69). He mentions the need for improved temperature compensation to reduce drift.
During World War II, the HRO's were used extensively. The HRO-5 is the octal tube version. The Germans and the Japanese each copied the receiver and its now famous PW dial. See the January 1946 Radio News, page 98, for a picture of National's chief engineer comparing the German version to National's own. PW is believed to stand for "precision worm" denoting the gearing used in the mechanism.
The serial number on this example is O 321 . It has the later gray chassis-paint and rectangular IF transformers. According to an excellent article written in the Antique Wireless Association's "The AWA Review", Volume 4, 1989, the "O" serial numbers were not known to exist. The serial numbering, if in the alphabet order described in the AWA Review article, would date this set in 1936.
The article has been reprinted on the following web site.
AWA Review article Dating the Early HRO