This is one of three different over dates in the entire series. The Mint produced an over date for the circulation strike issues in 1862 and 1869. For the Proofs it struck over dates in 1863 and in 1869.
The 1863/2 Proof Three Cent silver over date has slight traces of a 2 digit underneath the 3 digit. The 2 digit can be seen mostly going from the lower portion of the 3 serif to the middle of the 3. The variety seems to be very scarce with possibly no more than 30 -50 coins in existence.