Unless that's not an OEM rad, it's a white plastic (or maybe red/orange) screw with a cross (phillips) screw fitting, very large. Bottom edge, on my W124 it's on the passenger side. Sometimes hidden behind a crossmember, there will be a hole to get to it from underneath. Not on the side if the bottom tank, on the bottom surface.

Very nice, as the coolant runs straight down instead of out sideways and onto something.

Peter

__________________
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!