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The Bull rim was obtainable in Season 1: "New Beginnings" by reaching level 6.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Bull's current MU count is at ~172, which is similar to most other rare items (such as Deathray or Flamethrower), and even lower than some rare items (such as Ice Spike which is at ~200), despite Bull Rims being at uncommon.