I bought a 750K with an impossible frame number.
Someone in the US had ground off the numbers and stamped their own version on.
The bike (which displays the attributes of a K4) came with a NOVA and full receipt from the importer.
The trouble is, that because the number on the frame doesn't relate to any CB750 that has ever been built, I can never get a dating certificate for it.
Never mind, I got it cheap, and another £300 bought me a K1 frame with a NOVA