Hello,
There are only TWO waterpumps in the generic suppliers(TRW, Quinton Hazell, etc.) lists for ALL 4 and 6 cylinder Mercedes engines from the M121/OM621/M114/M130 of the fifties/sixties until the M102/103/OM601/603 replaced those venerable engines.
The one you need will measure approx. 88-90mm from the mounting flange to the fan hub, being described as *long neck*, the other is a shorter(45mm?) neck unit.
Hope that helps.

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Nachi11744