What is the parameter that is passed to the mounter script on restore?
It is the base name of the current archive descriptor, without .gz extension (even if compression is enabled). For example, if the configuration file is /home/joe/sync2cd/Music and restoring needs archives 7 and 12, the script will be called first with Music.0007, then with Music.0012, and finally with Music.0000 (the 0th descriptor indicates the end of the restore operation, and allows e.g. ejecting the last medium).