1983 Diesel Oregon Parts?

https://medford.craigslist.org/cto/d/1983-ford-250-turbo-diesel/6622100085.html

No clue on the "affadavit" part.

Seller (or even previous owner to that) lost title, so Affidavit of Loss/Release of Interest (Form TD-420-040) is used upon sale to exchange ownership interest.  You can send it in, but if you visit one of the WA DOL franchises, they can process a rush for an extra fee:

https://www.dol.wa.gov/forms/420040.pdf

Medford is almost as far south of WA as one can get without rolling into CA, so paperwork might take a bit, but for wonky situations I always try to go in person to transfer title and registration.  I'm sure there is a good story behind the WA -titled truck in southern OR.  OR has very inexpensive yearly tag fees, so it's not uncommon to see, but it's prudent to check with WA to ensure back fees are not due to avoid a surprise and they refuse to transfer title without payment.  I'm seeing any tags at all on that WA plate, so one should be cautious.

The 1983 6.9 block and cylinder heads are known for cracking. The Hypermax turbo kit is one of the worst designs of all the aftermarket add-on’s. Factor in that the truck does not look to be well cared for.... it’s priced right for a running work truck but I wouldn’t buy it with any intentions of rebuilding it.