92rolls, thanks for the images with "_ ?". We love solving a good mystery.
Please take a minute to look at your images as they actually appear in the thread so you can see what we see. We can see something at the underlined area but they are too blurry to tell what it is. The closeups will need to be clearer.
It was mentioned by you and at least one other that when something is on multiple coins it is (usually) on the die . I added "usually" because MD can almost look exactly the same. MD would not be on a die but a mechanical issue showing on the coin.
When you mentioned "oily" could it actually have been extremely shiny areas?