If you are interested in buying a W123 300D - this one or any other - then now is the time for you to understand that the odometer, by itself, is not a reliable witness to the miles that the vehicle has traveled.

They have a tendency to eventually start "slipping", at which point miles cease to reliably be added to the reading. Once one has repaired such a problem (instructions available here and elsewhere), it should be obvious that it would be a simple matter to add or subtract miles from the odo pretty much at will.