You are correct that is the tank strainer, or should be. I pulled it out of my 76 300D a while back and found that it was just a plug, someone had removed the strainer from the bolt.

Usually you don't need to replace the strainer, just remove and clean with some gas or other solvent.

I dont think there is a seal on the plug, but im not 100% about that. Try just tightening it down to stop the leak.

Its a large allen head and i didn't have the correct size to do it properly. So I got a bolt the correct size, put to nuts on it and tightened them together and used that to remove the strainer plug. Hey, it worked.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg