Well I never have seen a 123 body with a mechanical tach for sure. I have seen several in the junk yard (non turbo) that have this plug. It just leaks a LOT of oil when running.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
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I love the smell of diesel smoke in my hair