When the current state highway numbering system was first posted in January 1964, SR 251 started at Jct SR 25 in Northport and headed NE to the US-Canadian border, where SR 251 ended and BC 22A began. SR 251 was dropped from the state highway system around 1983 or 84. SR 25, which connects with BC 22 at the border, had been PSH 22 before 1964. SR 251 had been SSH 22A.
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