1912 LONDON CONVENTION AND 1912 RADIO ACT
The U.S. Congress finally passed a comprehensive Act to Regulate Radio Communication, which was signed by President Taft on August 13, 1912, and went into effect December 13, 1912. Officially this new law implemented provisions of the 1906 Berlin Convention. However, a new International Radiotelegraphic Convention had been signed in London on July 5, 1912, to become effective July 1, 1913, and the new U.S. law included many provisions which actually reflected standards of the soon-to-be ratified London Convention, most importantly the requirement that most radio stations had to be licenced, plus the provision that radio operators now had to quality for operator's licences, not just certification. A number of publications were issued as the Bureau of Navigation worked to implement its new responsibilities under the 1912 radio act, including:
Radio Call Letters: Edition May 9, 1913. (This was the only version of this document, as subsequent call letter policies were reported in the annual Radio Stations of the United States, beginning with its first issue of July 1, 1913.)
http://www.ipass.net/~whitetho/1913call.htm for a listing of all call
C = Standard,
L = Limited,
R = Restricted,
X = eXperimental.