In January 1964, SR 603 began at I-5 Exit 63, headed west a couple of miles to Winlock, then north to Napavine, then NW, ending at Jct SR 6 a couple of miles west of Chehalis. Effective April 1, 1992, SR 603 from Winlock to Jct SR 6 was dropped from the state highway system. SR 603 from I-5 to Winlock became part of SR 505. Former SR 603 in unincorporated Lewis County is now called "Highway 603".
Before 1964, SR 603 had been SSH 12E.
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