On my 1968 280SL the air filter canister is held in place by
one double-threaded rubber buffer and a bracket attached to the fender wall. In a 3-way discussion with Naj and Dave Gallon we discovered that the bracket needs another special grommet with a central sleeve that prevents over tightening the grommet. The original grommet is nla and the current one supplied by MB is slightly different, that is, a bit conical on one side and not flat on both sides as was the original, but it should work. The grommet, w/o the sleeve, is available from Dave for about $2.10 (+shipping). If you lost the sleeve here are its dimensions: OD: 12 mm, ID: 9 mm, length: 6.25 mm.
Below is a composite photo (individual photos were provided by Dave Gallon) showing the bracket, original and currently supplied grommet, and sleeve.