I'd have say about the same as without the error.
I remember an Unc $50 1937 with heavy 90% offset printing on back selling for under cat. (without counting the error) last year. I bought a nice EF $10 1937 with a pretty substantial ink smear at the back for under cat. as well.
This error looks minor so you'd be lucky to sell for a premium I think, even with the Osbourne signature. I don't have a $2 ozzie but I would be much more inclined to pay extra for a high grade note than one with a minor oddity. Still a nice find though if you're into those kinds of notes.