I know Dylan is a difficult autograph to get as he won't generally sign things.
From looking on the internet there also seem to be so many variations on his signature and I remember seeing a BBC documentary from the 80s where he was signing with his left hand, and then I've seen photos with him signing with his right.
I have heard the same about Bob switching hands, and I have a nice authentic sample pool of Bob autographs which show a good deal of variation. But I agree with the consensus everyone else had. Not likely genuine.