Vacuum Box or Vacuum Modulator?
 

I'm new to Mercedes. Just got an 81' 300D that I'm fixing up and have fallen in love with. Neat car. My question is this: Been having transmission shifting problems and ordered a vacuum modulator from the dealer (at least that's what I told him I needed) and the part came in today. The package says, "Vacuum Box" and carrys part number A 123 270 06 79. On the back of the package it says, "Vacuum Box, Injection Pump." It sure looks like the vacuum modulator to me and is threaded on one end, the plastic part is red in color, comes with a nylon snap on cap, o-ring, a c-clip, and a little adjusting wrench. Is this IN FACT the vacuum modulator for the transmission? Seems to me it has nothing to do with the injection pump. Can one of you veteran mercedes types fill me in?