Third key, what is it's true purpose?
 

https://cimg3.ibsrv.net/gimg/www.mbw...db362ed2e3.jpg
As far as I've been able to find, it's called an emergency key. Maybe to be kept in your wallet in case you get locked out? Anyone have an official explanation?

How you choose to use it is up to you. If it serves the purpose, that's great, right? But keep i mind it does not possess
all the features of the fully equipped smartkey

Smartkey: ignition key, remote open/close roof and windows, remote unlock door, unlock trunk, blade key unlocks and super locks trunk, blade in trunk un/locks side doors (gas door), blade key un/clocks passenger door, blade key un/locks glove box, panic button turns on/off alarm

Flat key: can only operate the features in black font

(description above may vary depending on car/key model)

Raymond - that's awesome documentation. Is there a comprehensive source where I could download all the pdf's?
Thank you

try here. scroll down to MANUALS

and if you need more, let me know. but include year of your car and I'll search my stash of saved information

https://www.benzworld.org/threads/0-...order.1720154/

Thank you for your guidance. As time goes by and I see documentation and/or parts I download them or take note of remaining suppliers and repair facilities - esp. in the Atlanta area in anticipation of maintenance needs for my older 1999 E320.
Thank you again.
Dave

ahh....I have done the same over the years, for my 1999 generic E320 sedan. manuals, product brochures, TSB, MB service data. I've not brought my car into the dealer (2 miles away) in over 10 yrs. the odometer is just under 68k right now

if you're an information hoarder let me know and perhaps I can pass on some of what I have

It's likely some type of "valet" key. I had one for my first W201. It didn't work all of my car's features like setting and unsetting my car's security alarm which I learned the hard way early one Saturday morning.