Transmission Vacuum Part

[rmmagow]

85 300D
there's a blue canister on the drivers side fender under the hood. Two black rubber connectors coming from the top. One is stamped TRN. One of the black rubber elbows connects to a small green plastic piece, Pierburg 7.20841.00 and 5065 (looks like heat branded in number).
The part does not act as if it were a one way valve. It does hold vacuum if one holds a finger over one end and sucks on the other.
What is this thing? is it supposed to be a one way valve or is this some kind of orifice control? The other end connects to a long black vac line leading to the back of the engine and I am betting to the transmission.

Thanks.