The H9DC0 and H9DC ARE Bosch Super spark plugs, but parts stores usually only stock or can order the resistor version, HR9DC. the H9DC0 has a "heavy duty" electrode and replaced the H9DC as OE circa '86, but the H9DC is still approved by Mercedes.

Again, the issue is RESISTOR versus NON-RESISTOR, and there is no other line of readily available non-resistor plugs in the US other than the Bosch Super.

Duke