That's the electronic control unit for the feedback control (KE-Jetronic) fuel injection.
It uses the signal from the air flow meter (not purely mechanical as in the K-Jetronic) to determine mixture by controlling the current to a leak controller (the ElectroHydraulic Actuator) on the fuel distributor.
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!