The correct screw is under an antitamper plug in the little "stalk" that sticks up between the air flow flap and the fuel distributor. You will need to remove this plug if no one else has, and I don't remember how.
It is spring loaded up, you have to push down with a 3 mm allen (or is it 4? ) to engage it in the actuall screw on the fuel distributor.
With a long allen, you can make this adjustment through the hole in the top of the air filter housing -- this should give you a clue as to where it is.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!