Thanks for responding. That is indeed the only conclusion that the supplier of the images has come to. However, I'm not so sure, whilst there is some closing up of the marks because of reshaping and finishing following the application of the hallmarks, there is, to me, not enough to turn a 'C' into a 'O'.
Two other possibilities do spring to mind though, that some debris may have attached itself to the punch and caused this effect, or the possibility that the mark is an overstrike with part of the original mark showing through.
I'll doubt we'll ever come to a real conclusion over this one, but the images are worth recording just in case some new information comes to light sometime in the future.