The Caboose

In the second and third message (West 1 and East 2), the long isomorphs either have or don't have a space of two characters before another short gap pattern. We call this section of two characters (added to one message's repeated isomorph words but not the other's) the caboose.

This appears to be an infix of 2 letters added to the two occurrences of a phrase in one message but not the other. Whether it's actually an infix, prefix, suffix, or an extra two-letter word is unclear. For example, if it is a prefix, it could be something (in plaintext) like active/inactive or stable/unstable.