Using last 2 months. (Some untraceable sales may be older, I started working on this suggestion weeks ago)

Sale 1:

Sold for 4 keys

https://backpack.tf/profiles/76561199043177646#!/compare/1706572800/1706659200

https://backpack.tf/profiles/76561198122460993#!/compare/1706659200/1706745600

https://backpack.tf/item/3955886126

https://backpack.tf/item/11910858289

https://backpack.tf/item/13689240868

https://backpack.tf/item/10013987250

https://backpack.tf/item/7032914430 - history of keys (from the buyer's end) to make things easier. None of the metal matches.

= 4 keys

Sale 2:

Sold with 24.22 ref (~0.4) for 4 keys

https://backpack.tf/profiles/76561198850274253#!/compare/1706313600/1706400000

https://backpack.tf/profiles/76561198073016365#!/compare/1706313600/1706486400

https://backpack.tf/item/13555314055

= 3.6 keys

Other Sales:

#1

https://backpack.tf/profiles/76561198818081498#!/compare/1703980800/1705968000

https://backpack.tf/profiles/76561198165485707#!/compare/1701734400/1704153600

https://backpack.tf/item/12057973971

Untraceable/Bulk

#2

https://backpack.tf/profiles/76561198165485707#!/compare/1704153600/1704585600

https://backpack.tf/profiles/76561198971372410#!/compare/1704240000/1706227200

https://backpack.tf/item/12057973971

Untraceable/Bulk

#3

https://backpack.tf/profiles/76561198971372410#!/compare/1704758400/1704931200

https://backpack.tf/profiles/76561198453530349#!/compare/1704672000/1704758400

https://backpack.tf/item/12057973971

Untraceable

#4

https://backpack.tf/profiles/76561198044578541#!/compare/1700697600/1708300800 (won't update)

https://backpack.tf/profiles/76561198338304715#!/compare/1708300800/1708387200

https://backpack.tf/item/10508432043

Untraceable. Gambled afterwards.

#5

https://backpack.tf/profiles/76561199026390261#!/compare/1704931200/1705017600

https://backpack.tf/profiles/76561199069442401#!/compare/1704931200/1705017600

https://backpack.tf/item/7531414062

Untraceable

#6

https://backpack.tf/profiles/76561198327995316#!/compare/1704844800/1706832000

https://backpack.tf/profiles/76561198177872379#!/compare/1706918400/1707004800

https://backpack.tf/item/5819373236

Backpack purchase probably

#7

https://backpack.tf/profiles/76561198850121720#!/compare/1704758400/1704844800

https://backpack.tf/profiles/76561198399077482#!/compare/1704499200/1704844800

https://backpack.tf/item/3902051079

Untraceable/Bulk

#8

https://backpack.tf/profiles/76561199201937808#!/compare/1708473600/1708560000

https://backpack.tf/profiles/76561198453530349#!/compare/1708473600/1708560000

https://backpack.tf/item/10508432043

Untraceable

#9

https://backpack.tf/profiles/76561199510528049#!/compare/1708473600/1708560000

https://backpack.tf/profiles/76561199201937808#!/compare/1708473600/1708560000

https://backpack.tf/item/10508432043

Untraceable

#10

https://backpack.tf/profiles/76561198059382847#!/compare/1708732800/1708819200

https://backpack.tf/profiles/76561198047971773#!/compare/1708732800/1708819200

https://backpack.tf/item/5381897615

Dunno, bulk

https://backpack.tf/item/11391087712 - gambled

https://backpack.tf/item/8402550771 - unusualified

Marketplace Sales:

https://marketplace.tf/items/tf2/30616;5;u3006

$9.99, Jan 22nd

This sale will end up around 6 keys, which is way to high here. Excluding.

Sellers:

https://imgur.com/a/jKLMgGg

https://backpack.tf/item/12554467035 - ~4.35 keys

https://backpack.tf/item/4090325861 - ~4.35 keys

https://backpack.tf/item/10508432043 - ~4.45 keys

https://backpack.tf/item/5381897615 - ~4.45 keys

https://backpack.tf/item/3993833603 - 5 keys

https://backpack.tf/item/4051641740 - 5 keys

https://backpack.tf/item/5813059943 - marketplace

https://backpack.tf/item/12057973971 - marketplace

https://backpack.tf/item/9044467896 - marketplace

https://backpack.tf/item/7531414062 - marketplace

https://backpack.tf/item/5819373236 - marketplace

https://backpack.tf/item/4448722356 - marketplace

Sales at:

3.6

4

Taking 3.6-4 keys.