Marketplace Sales:

https://marketplace.tf/items/1015;5;u3006

https://manic.tf/keyprice/?at=2018-12-15 - $18.61 / $1.64 = 11.35 keys

https://manic.tf/keyprice/?at=2018-12-22 - $18.07 / $1.75 = 10.33 keys

https://manic.tf/keyprice/?at=2018-12-25 - $21.99 / $1.79 = 12.28 keys

https://manic.tf/keyprice/?at=2019-01-03 - $22.69 / $1.77 = 12.82 keys

https://manic.tf/keyprice/?at=2019-01-10 - $20 / $1.74 = 11.49 keys

X = 13

X = 12

X = 11 (2x)

X = 10

Other Sales:

https://backpack.tf/item/2642569298 (#1)

https://backpack.tf/item/7184219454 - Not unusual at the time

https://backpack.tf/item/6898007584 - marketplace

https://backpack.tf/item/1629992027 - tf2mart

https://backpack.tf/item/1481821136 - scrap

https://backpack.tf/item/1474072333 - scrap/marketplace

#1

SCM

https://backpack.tf/profiles/76561197961976244#!/compare/1546128000/1546214400

https://backpack.tf/profiles/76561198848933483#!/compare/1545955200/1546473600

Sellers:

https://i.imgur.com/hncSBdH.png

https://backpack.tf/item/1475404005 - 15 keys

https://backpack.tf/item/2141740872 - 20 keys

https://backpack.tf/item/5612916806 - 39 keys

https://backpack.tf/item/5461816904 - 39 keys

Sales at:

10

11 (2x)

12

13

https://i.imgur.com/T2Nq8hN.png - buyers at 10+ keys, excluding 10.

Going with 11-13.