These threads might converge
 

http://www.peachparts.com/shopforum/diesel-discussion/337841-another-passenger-door-lock-issue.html

I'm having passenger door lock issues with my 1985 300D. Sounds like it might be a similar issue.

The driver's door has the vacuum master switch...you have posted a picture of this device (black cylinder with 3 rubber hoses attached). Both rear doors have a mechanism that only allows lock/unlock based on the vacuum circuit.

The passenger front door has a vacuum control, but it also has a spring mechanism that allows for a manual override of the vacuum circuit (with the key or the inside plunger). Mine has never worked; when I took the door apart I found that some of the spring mechanism is broken/missing. Go look at my thread to see a picture of the top of the rod from the vacuum device. Your picture of the passenger door doesn't quite show the linkage; its where the top of this rod connects to a 90 degree bellcrank

thanks mike. indeed our threads are closely related.

in my photos i see a round spring on the driver's side which is absent on the passenger side. could this be the mechanism for allowing vacuumless key unlocking?

As I posted in "my" thread about this, I got my lock mechanism fixed tonight and buttoned-up my passenger door again (I hope third time is a charm). But before closing it up, I did check and both my passenger and driver's doors have that circular spring which is between the lock mechanism and the door latch. The spring seems to get involved when you are opening the door with the internal handle while the door is locked.

I made a new spring with some stiff wire; see my thread for details.