Reassurance shields on a green background seemed to be for the most part a WSDOT thing in the 80's up to the late 90's. I still see them posting a few of this style from time to time, but they have mostly replaced them with the black and white one-piece signs for state routes and US highways, and cutouts for interstates.
As I recall, this SR 531 sign has the old style fabrication sticker that WSDOT used through 1991. 1992 and up, they used a new style sticker with the last two digits of the year very large. Since SR 531 was created in 1992, and this sign has the old style sticker, I would venture to guess that this sign was posted circa 1991.