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Date registered: Mar 2013
Vehicle: 2008 ML320CDI 4Matic, 161K miles; 1983 380SL (die Rostlaube), 238K miles
Location: Fairmont, Minnesota, USA
Mentioned: 2 Post(s)
Quoted: 381 Post(s)
Not sure the key is relevant to the engine swap. All W210s have a metal key that unlocks the manual locks in driver's door and a few other places but not the ignition; the ignition is activated by an IR device in the end of the key fob you stick in the ignition.
The only real way to identify your engine, if it has indeed been swapped, is to find the serial number on it.
How do you know it started out with a V6? The VIN for an E320 (non-4Matic) starts with WDBJF65H, and the E430 starts with WDBJF70H.