Its hard to make recommendations when you can't clearly describe what you have. The metal line you describe sounds like the breather point for the altitude compensator (aneroid) as Dr. P stated. Nothing should be attached to it.
If you have an 80-81 you should only have vacuum shift control. If it doesn't exist then you either have a different tranny (earlier 300D trannies only had throttle/control pressure control via a metal linkage off the throttle to an arm on the pass side of the tranny) or you are running with NO control.
Verify which tranny you have as no diesel has power starting in second gear.
Bosch Master, ASE Master, L1
33 years MB technician