Zenith built a total of 7, 218 R-520/URR radio receivers for the Department of the Army in 1953 and early 1954, under defense contract number 15175-P-52. 

    This is a highly modified and ruggedized version of the popular H500 transoceanic. On the exterior, the most noticeable changes were the brown oilcloth covering, light brown wavemagnet, dark brown carry handle, antenna tip and pushbuttons, and, for the first time, a dial lamp. On the interior, changes included a removable metal shield around the fragile RF head, shielding on all the tubes, a fuse, and a pocket for the army technical manual TM11-877. 

    There were several other subtle changes as well. All the components were high quality, no paper capacitors were used, was supplied with a full set of spare tubes and fuses, and had color coded band markings on the dial and pushbuttons. Previous transoceanic radios that used the miniature tubes, and especially the 1L6 converter tube, were susceptible to fluctuations in AC power, so the zenith engineers added the 50A1 ballast tube to control this.

    The power options for this radio changed too. Included was the Z1 power adapter (was optional on the H500), which enabled the radio to be used in other countries on 220 volts AC/DC, and 4 different power plug adapters. For battery operation, three battery harnesses, designated W4 (red harness for internal batteries), W5 (yellow harness with permanently attached battery tube), and W6 (black harness for external batteries) were included so this radio could be powered by a wide variety of readily available military batteries. See the Power Options link for more information.

    Please e-mail me if you have any notes to add, or notice any discrepancies.

Reference:

The Zenith Transoceanic, The Royalty of Radios, by John H. Bryant, AIA and Harold N. Cones, PhD. Copyright 1995, published by Schiffer Publishing Ltd.

Technical Manual TM11-877, Department of the Army, January 1954