Message Starts

The first trigram value in every message is different, followed by large shared sections. The values of these initial trigrams are all over the place. The most common theory is that they could be numerical indices for the messages, like 1-9 to label each message, or possibly A-I if numbers are not in the plaintext alphabet. Another possibility is that they are not part of the message as a normal character, and are instead something like a checksum, or the last character of the message moved to the front, or something similar.