I live in WA too. I bought my boat from OR, but since OR doesn't have a requirement to register the trailer, my trailer had the old expired CA plates. My problem is that the CA registration did not have a weight for the trailer and WA needed an exact weight either from the previous title, or a letter from the manufacturer.

I was able to find out that SMP was Sweetwater metal products, but they don't do boat trailers anymore. I got a guy that told me an aproximate weight, then just made my own "letter from the manufacturer " using their logo that I poached of the internet. The other option would have been taking the trailer to get it weighed without the boat.