Book Question

This is the copyright page of my copy of Dorothy L. Sayers’ The Unpleasantness at the Bellona Club:

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The relevant information is “First published in Great Britain by Victor Gollancz Ltd. in 1921.”

The problem is, afaict this book was first published in 1928, by Ernest Benn Ltd. Cursory investigation indicates that Gollancz split off from Benn in 1927, so it may have had something to do with the publication; but it didn’t exist at all in 1921, and Sayers didn’t publish her first novel until 1923.

Does anyone know why this is in the New English Library edition?

(My copy was purchased in a used book store in Ontario in the 2010s, so that's not much help.)