Asked my indy about the plug.
The converters without a plug appeared the first time in Europ with the "updating" of the W210 (E-class) in september 1999.
Explanation of MB : to cut production costs. This plug is certainly
an expensive item !
The worst is when you drain and refill the tranny, almost 1/2 of the used oil stays in the converter !
You can draw your own conclusions on that...