69 chrysler 300 tail lamps

My 69 chrysler 300 is having two kinds of tail lamps. The ones which are nearest the center section are having only one button light socket and other are having two. I suppose those one button lights are brighter than the others so there is no brake/turn signal lights in these center ones? Is that the original set up or has someone of the previous owners modified the lamps?
Author: jymy