The so-called '60s peace symbol which, as you correctly noted, came from the 1958 "Ban the Bomb" movement in London, really has its roots in the Spanish Inquisition. It was painted on the doors of those who were enemies of the church/state and slated for execution. Its symbolism was that of an upside-down Christ on the cross.
The U.S. Government adopted a peace symbol of its own April 15, 1935, when Franklin D. Roosevelt and representatives of all 20 South American countries signed the "Roerich Banner and Pact of Peace" into law in ceremonies at the White House.
Promulgated by the world-renowned Russian artist Nicholas Roerich as a means to protect the world's art treasures in times of war, the pact called for a distinctive symbol--three magenta spheres in the form of a triad inside a magenta circle--to be flown on a white flag over all museums, historical monuments and cultural institutions in times of war. The symbology of the triad was of all arts, all sciences and all religions (or philosophies) united within the circle of eternity: "Peace through Culture."