I bought one from my MB dealer 20 years ago when I bought my first 190.

It's a combination wrench with short and long 14mm hexes at one end and a 17mm six sided box at the other.

The part number is 000 589 24 07 00

I don't remember the cost. It wasn't cheap, but it wasn't outlandish as I recall. It allows me to remove the fill and drain plugs from both the diff and the five-speed manual transmission.

Duke