This is an 4 ore Denmark stamp. It has an inverted frame, and is probably Scott 42a, which is watermark 112, or a small crown. The only other option I can find is Scott 42c, which is watermark 113, a slightly different small crown.

When I flip it over, under normal light, we don't see anything.

I put it on my flatbed scanner, so this is a scan from the back. I soaked in Ronsonol to try to pull out the watermark. What you may see is not a crown, but some marginal marks on the right side.

I tried to use Retroreveal, but this is the best that I could do.

Any idea what stamp / watermark I have here? Thanks for looking.