What is this on reverse of 1909 VDB? 0

[TON Collection]

It’s not  silver paint on the reverse because I thought that to.

[Greenstang]

Hard to say what it is without the coin in hand but whatever it is, it was added after the coin was minted so it is PMD.

If you want to try to remove it, give it an acetone bath and see what happens. It won't hurt the coin.

[JKK]

If I had to guess, I'd guess it was long in contact with PVC, like laying on top of a piece of it in that bar shape. Does that green stuff feel slimy? If so, acetone is the perfect advice because it will remove PVC slime, which otherwise would chow down on the coin's surface causing irreparable damage.