It seems to me that the laws pointed to and cited by the State website pertain to lands or areas that have been declared as historical or archaeological sites by the State.
The other section pertaining to "thirty years" pertains, it seems to me, to sites designated as historical that have been abandoned but are on State lands.
It doesn't sound like anything found that is thirty years older or more on someone's private property or even in a State park is effected unless it is on an abandoned tract of land that is under State ownership.
http://www.parks.wa.gov/publications...20brochure.pdf
This link takes you to the brochure and please note that coins in circulation are exempt from the entire statute. In the United States, all coins are legal tender and even if they are old are still considered coins in circulation. Even silver coins are legal tender and can
circulate freely for face value, although we know that the silver value is what we are interested in.
The brochure printed by the State of WA clearly states what is being protected. Historical artifacts.