The rank of acting major was chosen for similar practical reasons. A major was senior enough to be entrusted with the kind of documents Martin would be carrying, but junior enough that his existence would not be verifiable through the most obvious channels. There were a great many majors in the British armed forces in 1943. Not all of them appeared in standard reference publications. German intelligence could not simply look up Martin in published records and expect to find him, but the absence of such records would not itself be suspicious because the records of lower-ranking officers were not generally published.
The service number presented a specific challenge. Royal Marine officers had service numbers that followed a pattern, and an invented number could in principle be checked against records or identified as not conforming to the pattern. The number assigned to Martin was chosen to be consistent with the pattern for officers of his seniority and service dates, but it could not be independently verified against actual records by anyone who had access to those records.
The identity card was a central prop. British officers were required to carry identity cards in wartime, and the format and contents of these cards were standardized. A German analyst who examined the card and compared it to what they knew of British identity documents would be looking for specific features. The card had to be in the right format, on the right kind of paper, with the right stamps and signatures, and with a photograph that corresponded to the physical appearance of the man the card purported to identify.