The part number on the facia was never used in the parts catalogs or for reference in general, as it was never available separately. The early 300SD speedometers were 125 MPH, and the last 300SD cars had 85 MPH speedometers. This is when the U.S. department of transportation thought it was a good idea to mandate a national 55 MPH speed limit on all interstates across the nation. It was in part due to a fuel crisis, but also because the people running the DOT at the time were ancient and couldn't grasp the concept of new cars being able to safely travel at high rates of speed.
For the 300SD speedometer, there were three part numbers for the speedometer.
125 MPH was 008-542-91-06 (up to ID number 014083)
85 MPH was 009-542-24-06 (from ID number 014084)
200 KPH (for Canada) was 008-542-92-06.
And, as Lucas stated, the 125 MPH one was used for other 6 cylinder variants, as well.