When a new edition was released, the cryptographers were forced to start again. The original JN-25A system replaced the 'Blue' code (as Americans called it), and used five-digit numbers, each divisible by three (and so usable as a quick, and somewhat reliable, error check, as well as something of a 'crib' to cryptanalysts), giving a total of 33,334 legal code values. To make it harder to crack a code value, meaningless additives (from a large table or book of five-digit numbers) were added arithmetically to each five-digit cipher element. JN-25B superseded the first release of JN-25 at the start of December 1940. JN-25B had 55,000 valid words, and while it initially used the same additive list, this was soon changed and the cryptanalysts found themselves entirely locked out again.
Overall, this is an excellent start to a compelling essay. Keep in mind that although the committee allows you to write up to five pages, three double-spaced pages is usually adequate. Feel free to add more detail, but make sure your text is concise and transparent. You might consider "writing" ideas and then "rewriting" them. Too often, authors put ideas onto the page, but do not render those ideas compellingly. Giving your sentences a bit of extra effort, and giving your experiences a bit of extra reflection, will result in a persuasive essay that compliments your accomplishments and character.