Sale 1:

Sold for Smoking Pencil Pusher (Mini at 18.5 below)

No outpost

Seller: http://backpack.tf/profiles/76561198062802524#!/compare/1476576000/1476662400

Buyer: http://backpack.tf/profiles/76561198069090237#!/compare/1476489600/1476662400

X = 19

Sale 2:

Sold for 17 keys

No outpost

Seller: http://backpack.tf/profiles/76561198111521010#!/compare/1475884800/1475971200

Buyer: http://backpack.tf/profiles/76561198018719204#!/compare/1475884800/1475971200

X = 17

Sale 3:

Sold with ~10 keys in junk for Killstreak Australium Wrench (21)

No outpost

Seller: http://backpack.tf/profiles/76561198137122597#!/compare/1471392000/1471564800

Buyer: http://backpack.tf/profiles/76561198093334319#!/compare/1471305600/1471392000

X = 11

Smoking Pencil Pusher Mini:

http://backpack.tf/suggestion/5811a4c1c440453efe5a81e1

Sellers:

18 keys:

http://i.imgur.com/JDFeiGc.png - 11 days

http://www.tf2outpost.com/trade/28547769

http://backpack.tf/u/76561198042938501

20 keys:

http://i.imgur.com/OXgG59Z.png - 2 months

http://backpack.tf/u/76561198125791160

Sales at:

11

17

19

11 is obviously an outlier

Going with 17-19