For the 1913 penny there are two varieties consisting of two obverse die states
paired with two reverse dies. One obverse type has a die crack running through the letter E in 'REX.'
For the reverse dies, the difference is in the letters 'ES' in 'STATES'.
For the first type, the S is slightly lower than the E.
For the second type, the ES letters are in alignment.
Additionally, the letter E is in a deteriorated state.