A picture would greatly help.   Many times, the options ordered when new will determine what ports and port types are available.  Likely this is the case and its a cover for a blank (not used) option. D420 rear panel, from Dell users guide: [IMAGE]  
ok so on my latitude D420 on the side of the back with all the ports are it has a black square and its plastic it's just plain black it's not a port ether what is it?
the black rectangle