The link below may be of interest you and I can only assume this is what you were hoping for. However, all you appear to have is a common 1968 Philadelphia dime, which is not a proof, and is perfectly normal without a mintmark. http://www.pcgs.com/top100/coin8.aspx
What 'Books' said...... It's every modern collectors dream to find one of those proof coins Tito. Welcome to the forum......