A very worn die struck this coin, but there's also a very unusual die clash visible under the hair and at the back of the neck.
http://www.maddieclashes.com/adc-100c-1979d-02/There may also be evidence of the clash on the front of the face, especially between the mouth and IGWT, but better photos are needed to confirm.
Very nice find considering how few die clashes there are in this series.