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The Guitarriff was obtainable in Season 23: "Back to the 80's" by reaching level 7.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Guitar Riff's current MU count is at ~196, which is similar to most other rare items (such as Bloxxer or Beachball), and even lower than some rare items (such as Ice Spike which is at ~200), despite Guitar Riff being at uncommon.