Sale 1:

Sold for 38 keys

Seller: https://backpack.tf/profiles/76561198011321852#!/compare/1555632000/1555718400

Buyer: https://backpack.tf/profiles/76561198046923201#!/compare/1555718400/1555977600

https://backpack.tf/item/6913889249

X = 38

Sale 2:

Sold for 38 keys

Seller: https://backpack.tf/profiles/76561198313364985#!/compare/1558483200/1558569600

Buyer: https://backpack.tf/profiles/76561198071429938#!/compare/1558483200/1558569600

https://backpack.tf/item/220919125

X = 38

Sale 3:

Sold for 37 keys

Seller: https://backpack.tf/profiles/76561198104168501#!/compare/1554940800/1555113600

Buyer: https://backpack.tf/profiles/76561198084447725#!/compare/1555027200/1555113600

https://backpack.tf/item/149287690

X = 37

Sale 4:

Sold for 35 keys

Seller: https://backpack.tf/profiles/76561198007792525#!/compare/1553990400/1554076800

Buyer: https://backpack.tf/profiles/76561198062457737#!/compare/1553904000/1554076800

https://backpack.tf/item/220919125

X = 35

Other Sales:

https://backpack.tf/item/149287690 (#1 & #2)

https://backpack.tf/item/220919125 (#3)

#1

Bulk for Vivid BoA

https://backpack.tf/profiles/76561198084447725#!/compare/1558310400/1558396800

https://backpack.tf/profiles/76561198313364985#!/compare/1558396800/1558483200

#2

Sold for Singular-spelled Modest + 2 keys

https://backpack.tf/profiles/76561198313364985#!/compare/1558396800/1558483200

https://backpack.tf/profiles/76561198062457737#!/compare/1558310400/1558483200

#3

Sold with 2 keys for Vivid Tundra

https://backpack.tf/profiles/76561198071429938#!/compare/1558483200/1558569600

https://backpack.tf/profiles/76561198144227819#!/compare/1558483200/1558569600

Can be used on the Tundra, given the abundance of pure sales here.

Sellers:

https://i.imgur.com/apdA48H.png

https://backpack.tf/item/220919125 - 40 keys

https://backpack.tf/item/270109208 - 42 keys, duped

Sales at:

35

37

38 (2x)

Going with 35-38.