Unfortunately many of the Threepence's have die cracks and the like, probably because the dies were small and so damaged more easily. I have many that are not in the catalogues as such, so mine aren't of any significant extra value unless someone is nuts about those types of things.
But yours appears to be a much larger flaw, more likely due to a die crack breaking further. Good to find them whether they are worth much more or not.
Rob.