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That is, in fact, a Light Brown Leghorn - which explains your white eggs. ETA - the "tells" are the white earlobes and the comb. While a Welsummer would also have the single comb, the leghorn takes that comb to the extreme with that large, floppy comb you see on your bird.

Do you have a photo of the bird in question - it may be that your Welsummer is actually a Light Brown Leghorn - very commonly confused breeds....and the LBB lays a white egg as opposed to the dark brown of a Welsummer