For Volkswagen Jetta, Golf and New Beetle TDI on the A4 chassis with the ALH or BEW.
O-Ring Seal between 3-way cooling flange that attaches to the back of the cylinder head over the transmission. This flange is metal on manual transmission vehicles and plastic on automatic transmission vehicles.
On manual transmission cars, there is another o-ring between the three-way coolant flange and the coolant glow plug flange, available here: Coolant Glow Plug Flange O-Ring (A4 Manual).