Inside the tar archive there is a file named backup_information.yml. There is the information you are looking for. To find the version number:
tar xf 1411831804_gitlab_backup.tar -O backup_information.yml | grep gitlab_version | awk '{print $2}'
Where is the 1411831804_gitlab_backup.tararchive in question.
You can also make a simple script and navigate to the backup folder as follows:
for archive in $(find /home/git/gitlab/tmp/backups -name '*.tar'); do echo -ne "$archive - $(tar tf $archive backup_information.yml | grep gitlab_version | awk '{print $2}')\n"; done
Where /home/git/gitlab/tmp/backupsis the path to the backup directory. For Omnibus GitLab this is /var/opt/gitlab/backups.
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