Just about the only evidence I could find on the Internet to corroborate this question was on Foods of England, which simply describes them as "white wheatflour dumplings with apple."
I did discover (elsewhere) that a Dorset Knob is a kind of biscuit. (One of the "10 signs you've lived in Dorset a long time" is "You've stopped sniggering when you hear the words Dorset Knob.")
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