Right now there’s a Yater twin fin for sale on Craigslist in LA for a reasonable $375 (pending, of course, a closer examination of the board for any soft spots, delam, etc.)
The board looks like it’s in pretty good condition. It has also had some work done. You can see the repairs on the bottom (see the second picture above).
What’s curious to me, though, is that I’ve never seen an example of a twin fin design from Yater. With the swallow tail it almost looks to be a standard fish design. If I had to guess, I would say this may have been made sometime in the 1980s, but I can’t be certain. There’s no mention of a twin fin on the history section of Yater’s website, either. I’d love to learn more if anyone has any info!