This German pulley is manufactured by
INA, the OEM supplier to Mercedes. This is the identical pulley that is
available at a franchise dealer.
Some owners may feel more comfortable with the smooth pulley however,
either the ribbed pulley or the smooth one will perform properly in the
upper position. The choice is yours.
Grooved pulleys are designed to interface with the ribbed side of the belt.
Smooth pulleys are designed to interface with the back or non-ribbed side of the belt.