Not that unusual, i've had a cat B sold as spares only that was to good to break up so i just plonked another frame onto it, and a french import that i was getting a load of grief over at the DVLA so i did the same again.
loz also had an 1150 ADV that was an old 1100 on the log book.
Nothing illegal about it, so long as you've all the necessary paperwork, and if you do sell it make don't try and decieve anyone.
Thats all fine and dandy, as long as the rear section is not from a stolen bike, what is or was dodgy is that the owner did'nt know he had a 1150

