Karnak knows the answer that will be written on the work order returned with your 586:

"Received gun. Cleaned it."

Maybe not, I used to have a teenage son, so I'm aware I don't know everything.

Powder/lead marks on the outside of .357 cylinders seem normal to me, even on old guns. The "rust colored substance" could be factory preservative of some type, but it's hard to know for sure.