In The British Typex cipher machine i added information from report FO 850/171 (mentioned in the book ‘Alan Turing: The Enigma’):
Countermeasures against cribbing
As an ENIGMA type device (with a reflector) Typex was also vulnerable to the plaintext-ciphertext attacks used by the Allied codebreakers against the German plugboard Enigma. In order to hinder such attacks several measures were employed, such as burying the address in the middle of the text, cyclic encipherment for short messages and insertion of random letters in the text.
For example report FO 850/171 ‘Preparation of telegrams: use of code words: cypher machines and traffic: teleprinter services: en clair messages. Code 651 file 1 (to paper 4968)’ (25) says:
‘When encyphering on the Typex machine, the encyphered version of a letter can never be the letter itself. This sometimes makes it possible to assign with absolute accuracy even a small number of words known or estimated to be in a message to the actual letters of the cypher version by which they are represented. To obviate this danger operators must from time to time press a key not demanded by the text of the message; the additional letters resulting will make the accurate fitting to the cypher version of a piece of clear text quite impossible. Such an insertion should be made on average once in every 10 words while the body of the message is being encyphered; it should be made on average once in every three words during the encypherment of the codress, the prefatory details and the beginnings and endings, whichever of the methods of encypherment in paragraph 25 is being followed; it should also be made on average once in every three words throughout very short messages when they have to be encyphered separately in Typex (see paragraph 27). The insertion should be made within words and not between them.’