My '91 300SE has a small hard vacuum line that inserts into a small hole in the large L-shaped rubber hose on the air cleaner lid. the other end inserts into the fuel pressure regulator. The purpose is safety; to suck fuel fumes into the engine if the fuel regulator diaphragm fails.
The large L-shaped hose plugs into the valve cover to suck filtered air into the crankcase for PCV operation. The black threaded cap "might" have been placed over the lamda adjustment tower (sticks straight up out of the middle of the throttle body) if someone made a lambda adjustment and didn't replace the ball seal.

DG