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The Windmill was obtainable in Season 18: "The Wild West" by reaching level 3.
Windmill is in an extremely similar situation as bull spoiler, being from a season where not many people played + being locked behind the season pass this item is extremely rare to find. Items from the same season such as tumbleweed also have the extremely rare tag. Suggestion made by: @janisz_.
Supply is widely distributed — no single holder owns enough to register as a hoarder.