Ai (or Ai Archiver, Ai Archivator) is a compressed archive utility, and its .ai file format. It is credited to E.Ilya.
The only version that has been located is v1.1, which is a 32-bit Windows console mode program. There was at least one earlier version.
The IDArc utility seems to call the Ai v1.1 format "Ai32", and it calls some other unidentified version of the format "Ai". The Ai v1.1 software, however, does not use the term "Ai32".
Ai files start with ASCII "
The third byte is possibly a format version number (0x01 or 0x02). The fourth byte is possibly a flag for "solid" mode (0x00 or 0x01).