that's kind of the beauty of the system. we actually have no way of knowing if it was just random fill.
maybe they were just random digits with an off by one error or some other problem with the symbol set missing one symbol.
or maybe the supposed fill messages can actually be cryptographically confirmed as authentic fill messages via some clever scheme (that the implementation of turned out to be buggy).
or maybe someone from some sort of field operations chain of command just slammed the table and said "my people are tired of trying to decrypt fill messages, i don't care, just cut the nines out so the field agents know if there's a message."
maybe they were just random digits with an off by one error or some other problem with the symbol set missing one symbol.
or maybe the supposed fill messages can actually be cryptographically confirmed as authentic fill messages via some clever scheme (that the implementation of turned out to be buggy).
or maybe someone from some sort of field operations chain of command just slammed the table and said "my people are tired of trying to decrypt fill messages, i don't care, just cut the nines out so the field agents know if there's a message."
that's what makes numbers stations fun. :)