You may be right in the case of later 126 cars, but I can tell you from experience that my wifes 83 280SEL came with a wad of keys. Only the ones with the red dot on them would set the alarm.
I also can tell you that on early cars with standard keys the only difference between a valet key and the master was the blank it was cut on. One could cut a master from a valet by using the right blank.
Bosch Master, ASE Master, L1
33 years MB technician