SSH 12E was added to the state highway system in 1937. This route began at Jct PSH 12 (now SR 6) west of Chehalis and headed southerly via Napavine to Winlock, then southeasterly to Jct PSH 1 (US 99) northwest of Toledo. In the mid 1950s US 99 was shifted from the old 2 lane route through Toledo to the present route of I-5 west of Toledo. SSH 12E was truncated to the junction with the realigned US 99. When the current state highway numbering system was first posted in January 1964, SSH 12E became SR 603.
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