I'm up for it being a pre-production bike, or a Euro 1976 YZ100.
The swingarm appears to be the longer 125X type - and the fact that you can't see the chain on the bottom, suggests that its got an X-type spring loaded tensioner.
The forks appear to be 34mm 125C forks, but with 125X style aircaps and fork boots.
Its hard to see, but the X-type cut-outs for the fork boots don't seem to be there.
OOOORRR... it could be a not-so-hotso resto. Although the forks are still weird.
the book quotes yamaha as its source for photos so id say the resto option is out.
Im going with pre production too. Now i want a white x as well as yellow one!
Also nathan you have a pm mate.