Info Needed

I've been lurking around a couple of weeks but now I have a question. I was
looking through some odds and ends and found something unusual, at least for
me.

A US # 518 when turned over has a light reverse image which is offset some
from the front image. Actually there are partial light images of two different
stamps with a gutter between the two.

So any info out there? Looks like the sheet was put on top of a sheet with
damp ink.

Thanks,

Pete