The engine serial number is located on the passenger side toward the front close to oil pan level. It should be fairly easy to find.

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2006 E320 CDI
1998 S320
1987 190E 2.3-16

1990 300D 2.5 Turbo(sold)
1987 300SDL Turbo(sold)
1987 300TD Turbo(sold)
1987 190D 2.5 Turbo 5sp(sold)
1985 190E 2.3-16(sold)
1984 500SEC(sold)
1984 230TE(sold)
1978 240D(sold)-Miss it!
1972 280SE 4.5(sold)