I don't see anything in your pics that would concern me.
The black streak in the first pic looks like oil from the head bolt.
Did you try to just wipe that off?
What does the head gasket look like aroud that location?
Rust in the head bolt threads is common on this engine.
Just run a tap through the thread and replace the head bolt if it is rusted.

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76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.