Sometimes there is no way to remove stains from coins. If acetone doesn't do it, maybe water will. If that doesn't do it, I'd just chalk it up to pareidolia. There is just no way that a 1974 quarter can be muled with a dime that was minted 30 years earlier.
The area on the reverse is just whatever was on the vise or flat surface that the cent was on when it was struck with a hammer or squeezed in the vise.