The M-138-A had 30 positions in the panel for the paper strips. These panel positions were called channels. In order to increase security 5 of these channels were kept empty. A different set of strips were removed for each message, based on an elimination table. The table specified which of the 30 channels in the panel would be empty. The 5-letter indicator at the start of the message would be used with the table to find out which channels would be empty.
The procedure broke up repetitions in the cipher text. From January ’42 it was required for all SECRET messages.
A 1947 report
shows a variation of this system, this time with the daily keys being
‘scrambled’ instead of being written consecutively as in the previous table. However
i don’t know if this scheme was adopted by the US authorities.
From May reports based on decoded ACr2 messages were issued by Referat 1 and this continued till July 1943, when use of the M-94 was discontinued by the US forces.
Ironically they found instructions for the M-94 (Field Manual FM 11-5) in a Berlin library after they solved it cryptanalytically.