On my 81 300CD There are two sheet metal type clamps that clamp onto the bottom rail of the battery case. The battery case has a protruding lip if you will at the bottom edge. You should see two threaded holes at the front of the battery tray. These clamps are about 4 inches long and a bolt passes thru the middle of the clamp into the tray. No wrecking yards in your area?

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.