Diapason pianos were manufactured by Kawai in Japan but licensed out and rebranded “Diapason”. I need to figure out if they are still being made today or whether production has stopped.
The quality should be very good as Kawai make very high quality pianos. Kawai even make the Boston line of pianos on behalf of Steinway & Sons.
The photo to the right shows a Diapason and you can see that the cabinet design is quite similar to a Yamaha U3 or U1.
Buying a Diapason can be a good way of getting a quality used Japanese piano without paying the higher price of a Yamaha. If you can live with the piano having a daft name and if you can find one that has been properly reconditioned then one of these may be OK for you. However, they are very rare here in the UK as there is very little demand for them because nobody has heard of them. So you might end up plumbing for a nice Yamaha U3 or U1 or Kawai upright after all.