Hmm, 21? More than Miami, KAW, Storms, Peace and equal the Logo and Heart?

https://ibb.co/j6DR8k

Also previous suggestion is based on the only sale. Spelled Stahlhelm 1 to 1. Two of them at 21 unsold in 4 and 5 months, since that time.

https://backpack.tf/stats/Unusual/Surgeon's%20Stahlhelm/Tradable/Craftable/33

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Sellers: 21 unsold in 2 months.

https://ibb.co/g4SxEQ

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Sales:

1.

Sold for Corona Australis. (12.3 at that time) + Spec KS Sharp Dresser (1 key) + Spec KS Capper (~2 keys) + Starduster (~2 keys) + Blitzkrieg Stickybomb Launcher FN (~0.5)

https://backpack.tf/profiles/76561198147714737#!/compare/1496620800/1502668800

https://backpack.tf/profiles/76561198163938639#!/compare/1496707200/1496966400

X1 = 18 keys.

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2.

B\O of 18

http://www.tf2outpost.com/trade/29254263

Sold for Blizzardy Storm Triboniophorus Tyrannus + 2 keys.

Outdated. Sellers at 14.3 (one week), 14.5 (2 months) buyer at 13.3.

Taking 14

https://backpack.tf/profiles/76561198122724760#!/compare/1499990400/1500249600

X2 = 16.

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3.

B\O of 19

http://www.tf2outpost.com/user/76561198168676105

Strange Aces Blast Defence + Sold for Red Bear Shotgun FN

https://backpack.tf/profiles/76561198168676105#!/compare/1499904000/1499990400

18 + ~5 keys = 23, taking B\O.

X3 = 19.

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4.

Sold for Searing Plasma Backbiter's Billycock + Strange Weight Room Warmer + 3 keys.

~23.

Have no B\O on Outpost\Bazaar. Too high regarding the current proposals.

https://backpack.tf/profiles/76561198170878742#!/compare/1499904000/1499990400

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Also have B\O of 18

http://www.tf2outpost.com/trade/29229121

Most probably sold for 11-12.

https://backpack.tf/profiles/76561198213231068#!/compare/1498694400/1498953600

https://backpack.tf/profiles/76561198070773303#!/compare/1498176000/1498953600

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Taking 16 - 19; totally excluding sale #4 with the support of current proposals and regarding other effects.