I do not think that YF 448 is on the Machine Register but I may be wrong.
However when I put this Reg No in on the DVLA database and Scott as the make it came up as a 596cc bike registered on 18/3/1927. The bike is currently unlicenced. This info is in the public domain. The keeper’s details are not available unless one is authorised. So the good news is that the bike still exists and the Reg No is still with the bike and has not been transferred.
It is still possible that John Underhill may know of it.
