None of my DAs(all base 533mhz) have it, but I've seen it before. From my memory, it generally shows up on DP models and the higher speed SP ones.
Remember that ALL QSs-even the lowest spec single 733mhz-had a fan in the same location, along with a connector on the logic board(rather than the process card) onto which it is connected. I've never seen the plastic cover on a QS(and I've had most of the variants) although the CPU fan attaches directly to the case and has a shroud to direct the airflow over the heatsink. I suspect as much as anything that the plastic cover on the DA is to serve as a fan attachment point.
Remember also that the DA really has more in common with the QS than any of its predecessor graphite case models.
It looks like you have an upgraded graphics card also. I'm not sure what the NVII means on the sticker, but the card I see looks a lot like a GEForce 4MX. You also have a SCSI that looks like the OEM Apple one, although it's also not listed on the sticker.