We call this the "dead pedal." If I remember correctly with the W124, I believe it's molded as one piece with the vertical, side "kick panel" plastic piece underneath the dash next to the e-brake pedal.
It is held on with three fasteners:
Two underneath the carpeting (one is shown in a photo, the other is further up under the dash), and one underneath the flexible trim piece along the door.
First of all, move the seat all the way back to give yourself the most working room.
The two fasteners underneath the carpeting should be just push-on nuts that are very simply removed. Take a flat-bladed screwdriver and pry under the edge of the nut, and work it back and forth and it should loosen. Do that for the second one up higher as well.
For the third one, it's a screw that is hidden. You will have to remove the flexible edging along the edge of the where door seals .... where it curves upward to vertical. The screw is located underneath this flexible trim about 1 cm from the edge of the vertical plastic piece. It should be a Phillips head screw. (In the first image above, I see that you have already removed this screw).
Then, there is a large hook molded into the back of the vertical plastic piece. You will want to CAREFULLY and SLOWLY wiggle the plastic piece straight back and out toward the seat, keeping it vertical. This will allow it to come free and not break this molded hook.
Good luck.
Cheers,
Gerry