Why we pivoted then
F6 was reopened for two items from the agent-cipher research (#78): the turning grille (F6.20) and a deliberate slip in the transposition (F6.21). Both are now closed, with F6.20's long free keys blocked by cost. F12 and F3 come next in the ranking but have no open item, so the next family with work is F2 (fifth), with F2.4. The ranking is unchanged.
What happened before
- 0199, 0201: a 10x10 turning grille, every one of its 10^15 cards, copied out by rows or by columns, then or before the table. No card fits K4 or a scrambled copy with a free key of up to 6 letters (rows) or 4 (columns), or with any of 200,563 key words. 0200 ran out of time and was rerun as 0201 with a faster search.
- 0202, 0203: one column read out of place or two swapped, around every keyword columnar the record had closed (about 400 million slipped routes): at the copies' rate, except the row reshuffle, where K4 led every copy in 6 cells.
- 0204: the same family against 40 fresh copies: 1 cell, as a quarter of the copies manage. Chance.
The change we believe is needed
- F2.4: nulls at both ends of the message, as SOE's Dutch section padded its messages. The end run moves nothing, so only the start run matters; periodic keys there are 0023's, so the new part is the progressive key on the moved known letters.
- Lesson kept from F6: estimate a run's cost on the slowest setting and at the workers' real speed, put planted texts first, and give every arm a clock check.