The car is delivered with 3 keys, one is a square head master key - the second is a round head valet key the third "reserve" key is just a metal blade which is a copy of the master key.
The reserve key is actually your spare emergency key, In the W116 and W126 etc this key was given in a nice leather pouch which had a lanyard and also was pocketed in the delivery documents (when benz gave leather bound stuff)
The round valet key does not operate the bootlid or the glovebox. It only opens the doors and starts the engine.
I would suggest you get a new master key from shahnawaz ltd. It will be made from the original masterfile of your car in germany and will be cut from key stock (very durable). Dont use "chaabi ustaad" keys on any mercedes benz unless you are a masochist because MB use safe quality metal in the ignition and door tumblers meaning you would require diamond tools to drill, regular drills will wont mark it.
btw - the new plastic key heads from benz are made from recycled rubbish found on the side of a road in rural china. They will break very quick.
There is a trick to long life - when you operate the key hold it from the centre or near the place where the metal blade sticks out. This doesnt twist the plastic too much. If you twist the key from the edge the plastic has to transfer all the torque through itself, you already know how heavy the steering lock mechanism is in a benz.