Quoted from the MATE beta announcement: Integrity and authenticity checks: Once you have downloaded an image, please verify its integrity and authenticity. Anyone can produce fake ISO images, it is your responsibility to check you are downloading the official ones. Please read and follow the steps at https://linuxmint.com/verify.php Link to the sums: sha256sum.txt Link to the signed sums: sha256sum.txt.gpg
I installed the MATE beta yesterday night, but didn't have time to use it yet. I'll check sources.list tonight and report back. But my guess is that each point release has its own repo, like seen here: http://packages.linuxmint.com/