Sale 1:

Sold for Max’s Head (~19 at the time) + ~3 keys in pure/items

Seller: https://backpack.tf/profiles/76561198339104273#!/compare/1512864000/1512950400

Buyer: https://backpack.tf/profiles/76561198156032540#!/compare/1512864000/1512950400

X = 22

Sale 2:

Sold for Bubbling Backwards Ballcap (Suggestion at 14) + 7 keys

No outpost

Seller: https://backpack.tf/profiles/76561198346118738#!/compare/1512086400/1512172800

Buyer: https://backpack.tf/profiles/76561198161635287#!/compare/1512000000/1512172800

X = 21

Sale 3:

Sold with ~5 keys in items for Stormy Towering Pillar of Hats (Suggestion at 21.5)

http://www.tf2outpost.com/trade/29475400 - B/O of 27

Seller: https://backpack.tf/profiles/76561198156032540#!/compare/1512864000/1512950400

Buyer: https://backpack.tf/profiles/76561198134266699#!/compare/1512864000/1512950400

X = 17

Sale 4:

Bulk for Circling Heart Coupe

Seller: https://backpack.tf/profiles/76561198148733839#!/compare/1511308800/1511395200

Buyer: https://backpack.tf/profiles/76561198346118738#!/compare/1511308800/1511395200

No current sellers.

Stormy Towering Pillar of Hats:

https://backpack.tf/suggestion/5a76dc5844325a0ec217f931

Sales at:

17

21

22

17 looks like a bit of an outlier, and I’m not comfortable using that sale in particular, as it favours the other item it sold for more than anything.

Going with 21-22.