[TJ]

expavr, I recently put Ca. '41 plates on my 41 pickup. I got the plates from my neighbor (he had them on a '41 coupe) who signed the release of interest form and gave me the plates and form. From there I went to DMV and registered them to my 41 pickup. I now have 41 passenger car plates on a 41 pickup. DMV charges me the extra weight fee on my annual renewal plus the YOM fee. You definitely have been caught in a bureaucratic problem that should have been so simple. Common sense does not reign supreme in these agencies.
Here's what I would do to bypass the system; If you have Washington plates on the tonner you are good to go as far as Washington state is concerned. You still own the California plates as they can be transferred from car to car or person to person. If they are still clear in the DMV system all you need to do to sell them is show the potential buyer they are clear and give the buyer the form that releases your interest. The buyer should be able to go to DMV with plates and form and put them on their vehicle.