I think last time I saw those kinds of slots, they were proprietary slots for onboard winmodem/audio that hold a winmodem/audio bracket. The rest is self-explanatatory.
-- Windows NT: n. 32-bit extensions and a graphical shell for a 16-bit patch to an 8-bit operating system originally coded for a 4-bit microprocessor, written by a 2-bit company that can't stand for 1 bit of competition.