I'm not familiar with the '94 SL320, but have purchased quite a few transmission filter kits for the 722.3 in my 126 car. They come with a copper and aluminum washer.

In my case, the copper is used on the converter drain plug; the aluminum on the transmission pan drain plug.

I suppose it's possible the filter used on your MB is also used on others that would use a combination copper/aluminum washer as mine does.