Numbers-matching engines and 'Tauschaggregat' factory exchange units

Replacement engines, transmissions and differentials are common, and Mercedes 'Tauschaggregat' (exchange-unit) data tags mark factory-supplied replacements that are correct but not original to the car. Buyers press hard for engine-number-to-data-card verification, and a seller who dodges the 'is it the original motor?' question is read as confirming that it is not.