So, it seems that the "PS hydraulic idle solenoid" was used on both automatics and manuals.
I do notice that turning my wheel can lower the revs a bit at times, but it's really nothing that I can't easily compensate for with the throttle in a manual 280SL if I have to.
Was this engineering overkill, or does the 250SL need it for some reason? I guess that it was used instead of an electromagnetic solenoid in 250SLs with AC, whether auto or manual.
Leaving the hydraulic solenoid in place and adding an electromagnetic solenoid onto the cross shaft bracket would be tempting in a car with AC.