Are you sure it's not just a 'show plate' put on for the event when he/she parked up? It appears to be sitting on top of a conventional sized plate and there's no record of any Yamaha with Y66 OXT at the DVLA.
If it is the actual registration then he's asking to get pulled by the plod because not only is it too small and spaced incorrectly, they absolutely hate plates over the top of plates as they spell out theft, cloning, false identity etc!
Just to add: It is on the DVLA records, but I couldn't find it at first as it is untaxed! It expired on 1st March, so I assume it was trailered to the ACE!