Further to the uncovering of several Mingmi scrambling schemes mentioned the other day, Ivan Sourvillo found another scheme from 1926: Pingmi (平密).
As before, the mathematical formula can be reinterpreted as renumbering the pages/columns/rows. Thus, "base 9 with the digits 1–9" is equivalent to page numbering "skipping any number containing a zero" in Chenmi (辰密) used in 1916. Reserving one figure may have been more common than I thought. I wonder whether there were other variants like "skipping 5". (Skipping two figures is unlikely because more than 8000 characters need to be covered.)
See the new section, "Pingmi (平密) in 1926", here.
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