No dank memes in this suggestion. Only two sales on clean hats that are in-date(<3 months). Well, i hope this will work.

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Sale #1 10th - 11th of Maymay

http://backpack.tf/profiles/76561198114269795?time=1431287091 before

http://backpack.tf/profiles/76561198114269795?time=1431327600 after

1:1 with Bubbling Cannonball. Histories match. 17*0.9 = 15.3 -> 15

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Sale #2 1st - 2nd April.

http://backpack.tf/profiles/76561198139703890?time=1427871600 before

http://backpack.tf/profiles/76561198139703890?time=1427958000 after

Sold for... woah.... 16 keys pure + over 12 keys in other items... wow. Obvious outlier, didn't have a outpost trade as well.

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Conclusion: 15 in unusuals, over 25 mixed. Taking 15 flat

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About other possible sales: One duped was quicksold for 14? on op recently, one sale is, again, an insane high outlier and 3 months old.