The Post-War Years, 1945-1975 “The Glory Years of Amateur Radio”
The Atlantic City International Telecommunications Union (ITU) Conference of 1947 (the ITU had changed its name in 1932), reallocated some call sign blocks, and granted a few developing island nations their own prefixes. Meanwhile in the US, the call sign districts were moved around to equalize ham populations.
During the war the Midwest , and West coast industrial centers had greatly increased the amateur radio populations in those areas. As a result, a new 10th call district formed for the central Midwest, allowing Wisconsin, Illinois and Indiana to have the 9th district to themselves. The 6th district was changed to encompass California only. The remaining states that used to be part of the 6th district (Nevada, Arizona and Utah), were moved into the lesser populated 7th district. As licenses were renewed, the new call sign districts were mandated, and often entire call signs changed as a result. A new call was assigned to denote the new district, but one “might” keep their old suffix if it was currently unassigned in the new district. If the suffix was already assigned to somebody in the new district, a new suffix was assigned as well. For example, pioneer Charles Newcombe, 6UO, in Yerington, Nevada became W6UO in 1928, but had to change to W7VOwhen the state became part of the 7th district in 1947 as W7UO was already in use. The rule allowing special call sign suffix dispensation lasted until 1978, when the systematic call signs program began. (More on that later.)
Also at this time US Possessions had own unique prefixes assigned, ie: KP4 for Puerto Rico, KH6 for Hawaii, and KL7 for Alaska.
More at EHam, history of ham radio