Should be on the rear of the intake manifold, probably right side, may be fairly far down there. You will have to take the air filter off and look -- it's a brass barb fitting, I think.
The 4.5L with D-jetronic has a banjo bolt and a steel line, so I suppose the 420 is different.
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!