Your imagination is playing tricks on you. It is not a mintmark; something probably hit the coin somewhere along its 48 years in circulation. A mintmark would be raised; the mark you pointed out looks to be incuse. Look up the word 'pareidolia'.
It appears the faint D could be raised. It's possibly a faint die clash. I don't think it's dramatic enough to command much if any premium. You might try www.varietyvista.com to see if there's a listing.
