Though I have a 126 cars, my fuel filter setup in the back is pretty much the same as the 260 or 300. The vast majority have 103 motors.

The fuel filter has a counter nut already attached to it on both sides. You simply go one way with the counter and the other with the fastener that connects to the filter. Piece of cake.

Here's a photo of the filter. Note the counter nuts affixed on both ends. You can click on the photo to expand for a better view.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1F81F1R0G1F81F1Z1X&year=1991&make=MB&model=300-SEL-006&category=E&part=Fuel+Filter