This is the Rear Main Crankshaft Seal for Mercedes 320, 32, 430, 500, 55, 63, 600 and 65 motors.
When these go bad you will get an oil leak comming out between the engine and transmission when viewed from below. You will need to remove the transmission to change out this seal. While you are in there we highly recommend re-sealing the Crankshaft Seal cover (which may be the cause of the oil leak in the first place). To reseal that cover you will need a tube of Mercedes Sealant and we highly recommend the Applicator gun for the sealant. You will also need to replace the transmission fluid that you spill when disconnecting the lines and potentially if you pull the torque converter away from the transmission. Fluid loss will be between 1-8 qts depending on if the torque converter and pan comes off and gets drained. We recommend doing the transmission service at this time also.