This is an odd one.

Sold for a Terror-Watt Hard Counter(10 Keys with Overpay, Outlier)

http://backpack.tf/profiles/76561198049043920?time=1434309790 -> http://backpack.tf/profiles/76561198049043920?time=1434351600

Sold for a Gifted Miami Nights Engineer's Cap(18 Keys with Overpay)

http://backpack.tf/profiles/76561198069326122?time=1434610800 -> http://backpack.tf/profiles/76561198069326122?time=1434708821

Sold for a Miami Nights Triboniophorus Tyrannus(23 Keys with Overpay)

http://backpack.tf/profiles/76561198044445538?time=1438585200 -> http://backpack.tf/profiles/76561198044445538?time=1438671600

http://www.tf2outpost.com/item/440,4002901210,30128,5 Belgian History

Sold for an Orbiting Fire Bone Dome(20 Keys with Overpay) and 6 Keys(26 Keys total, Outlier)

http://backpack.tf/profiles/76561198235913336?time=1438498800 -> http://backpack.tf/profiles/76561198235913336?time=1438671600

http://www.tf2outpost.com/item/440,4005821653,30128,5 Belgian History

http://www.tf2outpost.com/item/440,4005945215,30162,5 Bone History

Gifted Miami Nights Engineer's Cap mini suggestion

Sold with a Strange Widowmaker(1 Key) for a Disco Beat Down Cyborg Stunt Helm(19 Keys with Overpay) and a Bill's Hat(~2 Keys)(20 Keys total)

http://backpack.tf/profiles/76561198111024518?time=1433487600 -> http://backpack.tf/profiles/76561198111024518?time=1433487600

Using 20.

10 Seems low, although 26 seems high because of a current seller for 21 on classifieds. Two sales, 18-23 Keys should be fine.

Additional Proof/Counter-Proof/Constructive Criticism/Feedback is always appreciated.