Played a game of hexakill with Varus, whom I already had level 5 on for quite some time. Received an S after the game, for which I received a mastery chest. I did not however, receive the mastery 6 token, as I was supposed to.
Rank 6 & 7 can only be achieved on SR.
http://na.leagueoflegends.com/en/news/community/q/chromas-yorick-and-champ-mastery-ask-riot
As some of you have pointed out, mastery level 1 to 5 are as much about continued play on a champ as they are about skill mastery, great play just accelerates gains. Mastery level 6 and 7 have a higher competitive bar as far as performance requirements. Alternate game modes are intended to offer variety in play with a less competitive standard for players. We want to maintain mastery levels 6 and 7 as achievements specific to high performance in competition, so they will only be achievable in SR modes. - Riot Socrates
