Came across an interesting proposal to renumber the primary/secondary highways (aka the former number-and-suffix system) from 1957, which would have turned a few dozen secondary branches into primary routes. Highway 51 to Monroe! Highway 40 to Westport! Highway 56 to Moses Lake!
(https://i.imgur.com/OgQIF2n.jpg)
Is this from Washington's version of CH&PW?
It's from the March 1957 edition of Washington Highway News (found here at the WSDOT Library Digital Collections (https://cdm16977.contentdm.oclc.org/digital/collection/p16977coll1/id/2487/rec/8)).