I was at the National Archives in Kew recently with the intention of researching the British WW2 cipher machine TypeX. Firstly to see what type of message indicator was used, and secondly to find out more about the structure of messages enciphered using a TypeX.
Furtunately WO 208/5109 came up with a few answers.
Bearing in mind that these are setup and usage instructions for the TypeX Mark VI - which was a portable hand-wound derivative of the TypeX.
before the message is commenced, a five-letter message setting on the scrambler, as indicated by the letters appearing at the windows in the cover of the scrambler, should be selected and the corresponding disguised indicator noted at the top of the message form.
Using the left-hand while turning the handle with the right, type the message exactly as it appears on the message form, giving the address, serial number, date and text of the message, the space key being depressed between every word of the plain language message. The message printer automatically spaces after every five letters of cypher have been printed. At the conclusion of the message sufficient letters must be added to the cypher message to make the last group into a complete five-letter group. Therefore note the counter reading and depress the space key, letters shift key, or figure shift key or a combination of the these until the last figure shown on the counter is either 5 or 0 (both coloured red). No use should be made of other keys for this purpose as their printing in the decyphered message might possibly lead to confusion. Finally, stick the tape on the pad and write the message setting in the manuscript as the first and final groups of the message.
The last bit of this extract is the important bit - it details that when constructing a cipher message using the TypeX, the first and last groups of the message are a five letter indicator. Just like we see in our pigeon message.
Preparation of message for transmission – Withdraw and detach the tape from the message printer and insert in manuscript, as the first group, the disguised message setting used at the commencement of the message. If the message is complete in one section write this disguised message setting and groups of five Q's must be inserted appropriately in the spaces left for that purpose. Each section must begin and end with the disguised message setting, in manuscript, appropriate to it. The tape should then be gummed to a message form so that there are ten groups in each line. This is done in order to facilitate the counting of the total number of groups.
So, I am fairly confident in saying that we are dealing with a message enciphered using a TypeX. What makes this all the more interesting is that I cannot find reference that the TypeX had ever been cracked by the enemy. So for someone to break this code, it would also carry the prize of being the first to crack a message enciphered using a TypeX.
At the very least, we can be happy about the fact that it is unlikely a one-time-pad cipher, because then we truly could presume it indecipherable.