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Well, that's an interesting mystery. Culme, whose records go up to 1914, does not record the "C&H" mark so, as this is with a 1909 hallmark, we can probably assume that it was never registered. In addition Culme's history of Carrington & Co does not record any sort of partnership which might have given rise to a "C&H" punch. The interesting point for me is that the cartouche is the same, probably unique, shape as Carrington's "C&Co" mark. Maybe the punch maker got his initials mixed up and the punch slipped through without the error being spotted - that sounds very unlikely, though. My only other thought is that the base is a post-1914 addition, but in that case it should have a full hallmark including the later date letter.
I think we're looking at an overstrike. If you look at the second image from the bottom, a partial letter can be seen above the ampersand. The second mark image also shows evidence of overstriking.