alt Hacker NewsBefore keyless became defacto standard, most keys were fixed on a hinge - you'd first unlock your car by pressing a button on the fob, then swing the key open and use it to start the car.
Nowadays the physical key is only a backup, safely stowed inside the fob. It is meant to be pulled out only in an emergency.
Mercedes changed to the IR key in maybe turn of the 90's. The plastic blob would be used to turn the ignition like before, but the locking part was electrical (optical). There was still a metal key that could be pulled out and used on the exterior locks if the central locking was out.
When keyless start came, there was a dummy button that fit in the ignition lock that could be pushed to start. If there was a problem, you could pop it out and use the key as before.
IIRC, all that, so may have mistakes. Just crossed my mind that they went a few decades between no metal ignition key and keyless. MB being MB, I wouldn't be surprised if they still had models with that same "old" system and keyless as an option.