Here is a progression by rows, 340 on the left and an average message from my first experiment, with transposition, on the right. If a cell is colored, that means that it is unique to the region(s), whether top / bottom or middle.
13 1 35 3 26 32 58 26 14 21 3 50 56 33 60 7 5
30 41 17 44 27 18 38 68 8 46 19 22 34 38 48 16 45
36 30 28 6 7 40 52 68 49 68 9 1 20 42 47 48 2
24 36 61 3 68 68 32 25 26 31 59 27 59 29 59 43 14
5 17 41 8 38 60 17 57 61 18 39 6 20 7 60 44 68
59 42 45 33 55 11 30 34 44 12 1 2 62 51 62 32 28
20 9 35 3 4 40 8 9 68 17 18 35 31 43 53 20 5
6 29 7 46 55 63 40 39 17 14 15 34 34 68 34 21 10
16 52 55 37 45 40 63 33 30 56 11 25 59 12 34 68 38
68 8 32 57 68 68 34 9 17 26 45 18 13 30 39 29 30
10 14 8 35 34 47 55 1 45 58 12 46 15 61 63 36 2
37 3 4 19 27 31 60 68 20 16 68 35 68 4 44 49 61
22 48 32 23 59 8 36 24 6 7 29 37 45 35 3 63 36
37 33 44 56 68 18 28 45 19 1 33 40 41 2 3 47 25
20 4 31 9 68 35 5 36 62 53 32 60 38 31 6 13 33
39 37 4 35 28 17 19 68 34 50 40 19 68 29 41 68 51
58 49 18 43 22 34 33 68 38 24 40 17 18 34 52 27 68
6 10 2 44 30 62 68 3 9 30 28 61 25 12 36 41 68
4 53 40 8 29 26 9 24 19 32 38 42 3 59 6 39 30
20 1 14 23 62 31 29 23 10 55 33 52 11 12 55 30 45
You can see a big difference in symbols only appearing in the middle for 3-14-3. Almost twice as many. 34 for the 340 and 18 for the test message.
unigram distance 3.png
And then the difference starts to get more dramatic stepping into 4-12-4, 5-10-5, and 6-8-6.
unigram distance 4.png
They get more similar with 7-6-7, 8-4-8 and 9-2-9, except that with 8-2-8 there is a big difference between the top / bottom counts.
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