Marshall Hall devised [Halls weights] at a time when the indicator system had been broken, but just in case …. Its time did come when the Japanese introduced bigram ordinates to specify additive starting points in December Edward Simpson
30 November 1943 Tentative Method for the Alignment of JN-25 Traffic Without Use of Code or Keys. Lawrance Shinn Ray C. Hackman
My initial reaction to it was to wonder why anyone would go down that road. Collapsing five-figure groups into mod 10 sums of their digits loses an awful lot of information.
Differences of Scanning Groups Distribution of Differences