Older Benz models (like W123/6 and even the latest facelifted W124s) do not have central locking switches in the center dash. They had to be locke with the door lock (unlike newer models like the C/E/S classes).

The alarm equipped models just had a black rectangular fixed cap with a very small red L.E.D. in the center (the size of a switch).

A. Rosich
S320, 1998
E320T, 1995