SSH 12D was added to the state highway system in 1937. This route began at Jct PSH 12 (at the time US 830, now SR 4) near Cathlamet and headed upstream along the Elochoman River to the vicinity of its confluence with the West Fork of the Elochoman River. When the current state highway numbering system was first posted in January 1964, SSH 12D became SR 407.
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