Correct - that is a cover that was used for Mezzanine slot cards. There were no "official" cards for this slot - Apple never published the pinout, and never sanctioned any Apple-made or third-party cards.
However, there *ARE* cards that use it. My iMac G3 has a SCSI card, which uses the VHDCI SCSI connector. There was a Voodoo 2 video card, that necessitated the use of an external monitor for 3D-accelerated video, it's video port was there. Sonnet also made a CPU upgrade that put a FireWire port there.