Tonight stopped by local bank. Asked for some rolls of half dollars, to my surprise the had $40 worth. Did find silver but also some other cool stuff. 1st 1983 D
"pluribus" in motto? If so, it's because it is not a full strike. That location on the reverse die is one of the deepest portions and frequently is muted or partially missing. Thinner than normal planchets or wider than normal die spacing is the cause.
1983 D looks like machine doubling as the "shadows" run in the same direction on both sides if you use an "x-ray" view.
I think it's machine doubling. And the missing thing may be the FG, but I think the pics aren't good enough to tell if it's really missing or just worn down at that die.
Might be the "weak" one (where half is there)... you should read this and related threads, maybe: http://www.cointalk.com/t84111/ as I don't know too much about them.
This is why I joined this site, because most people are helpful and always wiling to help others learn.