'92 Cummins 5.9 IC'd, stock boost levels.

Leaking oil like crazy from drivers side. I've resealed the vac pump, and have a new billet tappet cover, and it was all degreased prior to a 10 minute freeway run, then I snapped this picture.

What is this plug for? I've never touched it. It appears my leak is coming from either that plug, or the head gasket junction right below it. Before I remove it, I'd like to know what it's for, and what is behind it. I can't find it in my service manual.