I've just been idly meandering around the internet where I discovered a very interesting bike: a 525cc Yamaha trials twinshock, built by a talented French engineer, Frédéric Fleury (?) - see link below.
Apparently it was based on a Yamaha XT500. Which reminded me of a photo I saw years ago in a Mick Andrews trials book (forgotten its title, but I remember the book was small which contrasted with the bigger books by Sammy Miller and Don Smith - I still have all three somewhere upstairs).
The photo was of a partly constructed 500 trials bike that Mick was building, based on an XT500. For years I wondered if that build was ever completed - apparently it wasn't due to Yamaha pulling the plug (as mentioned in Fred's article below), which was a shame.
Anyways, Fred invited Mick to have a test ride. Mick's very popular over here in France where he often attends trials gatherings. Apparently he gave it a thumbs up. It's probably not a very competitive bike but nevertheless it's an impressive bit of machinery.