After replacing my manifold-front exhaust pipe gasket on my 88 sedan (automatic), I noticed that there is a port of some sort on the engine bell housing on that side of the engine, about 2/3 of the way up the side of the engine, to the rear of the manifold. It looks like a connection for a 1/2" hose, or similar. It's not connected to anything, and I'm just curious what it is and (not being the original owner) if I should be concerned that it's not hooked up to anything. Is this a vent hole for a manual clutch?

Mr. Tool has it correct. It is the engine coolant drain.
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john

i belive it's the drain valve for the anti-freeze in the engine block, no concern unless it's leaking
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Mr. Tool

It puts heat into the engine when its cold outside

No, I think you're talking about the big flexible air hose that attaches right to the manifold and goes to the air box. That's not what I'm talking about. What I'm talking about is a small 1/2" or so port right on the engine bell housing, apparently going into the clutch/flywheel area.

I'm not familiar with the hole, but it may be the location for the crank position sensor on later LH2.4 & 3.1 cars. I believe there's a knock out for the sensor on all the bell housings, but I don't recall looking to see where it was or what it looked like since I've never needed it.

old engines as B21 had ignition timing sensor there. if you mean a hole, where you could see your flywheel (or touch it with a pin),have seen those on B230 too.
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i prefer non-german cars