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The Marrow rim was obtainable in Season 33: "Jurassic" by reaching level 6.
Marrow is the newest L6 reward from the season. Its comparable to other 250K limiteds such as (but not limited to) Belgian Waffle, Piggy or Trophy Wings, and is very much interchangeable with those (as well as other 250K items).
Supply is widely distributed — no single holder owns enough to register as a hoarder.