I can't tell for sure if there's a split on the top serif. Do you see one? If not, I'd say it is the broken D punch. There was a D punch in 1979 that was broken and resembles an RPM on the lower serif.
http://www.coppercoins.com/lincoln/diestate.php?date=1979&die_id=1979d1mm003&die_state=mds Lower serif looks like this one to me.
The one listed on copper coins has both upper and lower splits. Like noncents said this is a damaged punch.