Here is the base plate used in Washington from 1968 to 1981...
Washington License Plate ColorsThey were green on white...not yellow. However, it could be that personalized plates were yellow then, unlike the normal plates.
Personalized plates in Washington were first introduced in 1975...
License Plate HistoryIn order for these to be registered to a car...
1. Most likely only a 75 or 76 would be eligible under
Washington's Year of Manufacture license plate program due to the fact that personalized plates were not available for prior year cars.
2. The year tabs would have to be removed which may further damage the plates.
3. If the DOL does not have some documentation showing the year range of the early personalized plates and that they were yellow, I think you'd have a heck of a time convincing them that the plates are eligible for Year of Manufacture issue.
It would be a real crap shoot to get them registered to a car, but they would make great wall hangings.