Here is an interest tagging effect. Can you see the ghost of Paul looking over his shoulder? If you can't I have highlighted the ghost image by the red box. While looking at the tagging on this pair, I noticed what looked like an image. I ran RetroReveal and that help bring out the background image. I still could quite tell so I blurred the image slightly then ran it through RetroReveal a second time and the image became clearer to me such that I was able to see it is the image of Paul on the stamp itself.
Since the ink stands proud of the stamps surface, the impression most likely got pressed or worn into the tagging mat. At some point the tagging mat and printed image got out of sync and offset creating this effect.