280SL Engine cooling circuit

[Thierry du Laos]

When I drained the coolant circuit, prior to remove the engine. At the very end of the drainage I had a very little trickle of a greasy dark part.
What are most probably the origin of such stuff, according your experiences.
From looking at the circuitry, I see two ducts used for cooling the auto gearbox going through the bottom of the front radiator. Is it frequent to have rust problems on that part inside the radiator?

Thierry