...so that's what it called - "oil pressure test cover"! Thanks, Paul.
That's what appears to be leaking on my E320. Here's my recent thread about it.
http://www.peachparts.com/shopforum/showthread.php?t=134529
I haven't replaced it yet. I've been hoping to get some advice on replacing the gasket before I just try taking things apart. Do I need to drain the oil first? Do I just unscrew the torx screws, pull the cover off, replace the gasket and then screw the cover back on? Do I need to do anything with the plug?