Here's the Godbehere-Wigan image that I came across years ago:
It's a little surprising perhaps that they did not choose to strike the mark on a smooth part of the coin face, rather than on the King's head itself.
I doubt that these are counter-marks that are sometimes to be found on coins, the coin value being so low. Perhaps they denote when an apprentice is allowed to strike the maker's mark for the first time?
Hopefully other members may have further examples.