It sounded to me like it has less to do with the message content, and more that someone who'd worked with Rackstraw in the past thought the message was written

It sounded to me like it has less to do with the message content, and more that someone who'd worked with Rackstraw in the past thought the message was written in a way similar to Rackstraw's. Rackstraw served under Sherwood in two classified units, and Sherwood was familiar with his writing style having deciphered some of his earlier messages. When he saw the fifth and sixth typewritten letters, he immediately thought the “odd letter and number combinations” were indicative of the type of coded message that Rackstraw would send. or something