Seen a few of these. This has been the biggest of them all. I really don't know if its one or not. Maybe a drip of grease
I see a UFO unidentified flying coin object,looks like damage.It doesn't resemble a strike through to me,your grease theory is a good one.
The lower image has all the characteristics of a strike-thru. If the mark is into the coin's surface, that's what it is. Now, if a poster decides to describe exactly what that mark looks like to someone who cannot see it (Remember my daisy quiz?) they will stumble into the answer of what it is. PS You can delete the reverse image as it adds nothing to the discussion.
So is this just what strike throughs looks like on zilincons?the worst quality type because this isn't the look I usually see.
If you BOTH wish, try to describe what you see on the coin to me. It's easy and then you'll probably ID them easily from now on with a good image. It's all in the "details" of what is visible in the image.