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The Mecha Arm was obtainable in Season 8: "Looking to the Future" by reaching level 5.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Mecha Arm has become quite an overlooked item. As a result, it no longer on the same level of rarity as Jetpack or Armor and is now more comparable to Afterburner and Soccer, both of which are at very rare.