Yes, "switch-over valve" is one of the many quirky German translations. Even stranger, the similar 5 electric vacuum-valves in the climate system are termed "codes". Effectively, it is like a Single Pole Double Throw (SPDT) electrical switch. Describing its function can be clumsy since when a port is "open" that is equivalent to an electrical contact being "closed", so maybe term that state "on".

As I recall, that switch-over valve is only in later OM617 cars. I know it is in my 1985 300D and recall it isn't in my 1984 300D. I recall that it blocks the intake manifold pressure signal (Pman) from going to the vacuum piston on top of the injection pump under certain conditions. There are other switch-over valves in the EGR system.

1985 cars also have the "blue moon" vacuum-amplifier for the VCV vacuum signal to the transmission (modulator on side). I added one to my 1984 since seems like a good idea. 1985 cars also have the Klima Relay which disables the AC when the pedal is floored (and other functions?). A reasonable question is why throw those new things into the last year of the W123 300D. I recall both continued in later models, so 1985's were apparently guinea pigs.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans