Is that the 1974 edition? It does seem strange that the Mint should give false information, unless the relevant records and personnel had gone by the mid-70s. My research and article were done mid-90s. I would say that when the ESC were writing, the 1921 shilling with the pre-1920 obverse was not yet known about, but when you compare it with the normal 1921, the difference is obvious and yet both were struck from the same alloy.
In one sense the Mint was correct: it IS the same bust but with minor changes to lower the profile. It is not, as in the case with the ME, a new design.