I checked 4 different WEB parts providers including this sites Parts Shop and Fastlane. All show a "reference sensor" located at front crank pulley - sounds like a TDC sensor. The optional part number shown is 001 158 91 28.
All sites listed this part as being unavailable. Sounds like a dealer item.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.