That's a W140 part. The W126 uses a similar device attached to a heat sink with posts. Often called a porcupine -

http://www.carpartsdiscount.com/auto/viewer/pictures/63808/1000/1/P/2C496E4/mercedes_560sel_behr_heater_fan_resistor_oem_126_820_53_10_1268205310.html

Similar price too

US W126s from 1986 use the solid state contoller but I don't know if it's the same part through 1991. Earlier W126s use a resistor pack.

Sixto
87 300D