I can't make a suggestion for another hat due to multiple endless mini-trains, so might as well try my hand at this straightforward one.

It's a 1:1

With a strangifier used on it

https://backpack.tf/item/5194909141

Sale 1 BULK

It stayed in the unboxer's inv since 2016, was sold in bulk this Jan

https://backpack.tf/profiles/76561199100814466#!/compare/1611360000/1611532800

Sale 2 BULK

Was bought in bulk in Feb

https://backpack.tf/profiles/76561198083134207#!/compare/1611532800/1611619200

Sale 3 55 Keys

The was bought for 55 keys

https://feen.us/j9n28w.png

https://feen.us/ugs7bm.png

Since there is only 1 sale in pure keys that isn't part of a bulk, 55 Keys seems like the only option.