It's pareidolia. I can see a Greek sigma next to the "F" but that doesn't necessarily mean one is there. The "F" is also visible on the other stamp:
But note the upright of the "F" is slightly curved. Comparing the position of the set-off, the hairband and hair is what you are seeing, combined with the visible inked parts of the front side:
Posted 09/07/2019 12:19 pm
