You're a wizard, Harry.

Evidence courtesy of Erik (I'll link his profile later)

https://backpack.tf/profiles/76561198097022357#!/compare/1521763200/1521936000

https://backpack.tf/profiles/76561198310972287#!/compare/1521849600/1521936000

Sold for 20 keys.

https://backpack.tf/profiles/76561198260650009#!/compare/1523664000/1524009600

https://backpack.tf/profiles/76561198164440171#!/compare/1523836800/1524009600

Sold for an Infernal Smoke Taunt: Skullcracker. Likely better used there, would support the range.

https://backpack.tf/profiles/76561198305676157#!/compare/1525737600/1525824000

https://gyazo.com/93970900575bda2ef5ba031761533031

Sold for 19 keys. Dates match.

https://backpack.tf/profiles/76561198305676157#!/compare/1525996800/1526083200

https://gyazo.com/93970900575bda2ef5ba031761533031

Bot backpack hasn't been updated yet but sold for 19 keys.

https://backpack.tf/profiles/76561198305805743#!/compare/1522713600/1523145600

Unusualified.

https://backpack.tf/profiles/76561198156052965#!/compare/1524182400/1524268800

https://backpack.tf/profiles/76561198049054413#!/compare/1524182400/1524268800

I've counted the keys and and they all match. 18 pure.

https://backpack.tf/profiles/76561198217689741#!/compare/1523836800/1525478400

https://backpack.tf/profiles/76561198263801172#!/compare/1523836800/1523923200

Sold for a Galactic Codex A Head Full of Hot Air (9) + Green Confetti Fat Man's Cap. You're free to look into the fatman for a mini but I doubt it would end up at 8 keys flat or below, so this supports the low end of the range.

https://backpack.tf/profiles/76561198344933095#!/compare/1522108800/1522195200

https://gyazo.com/93970900575bda2ef5ba031761533031

Sold for 24 keys.

https://backpack.tf/profiles/76561198305676157#!/compare/1523059200/1523145600

https://gyazo.com/93970900575bda2ef5ba031761533031

Sold for 22 keys.

https://backpack.tf/profiles/76561198410650505#!/compare/1522195200/1522281600

Unusualified.

https://backpack.tf/profiles/76561198129197811#!/compare/1525996800/1526083200

Better used there.

https://backpack.tf/item/5778012740

Bunch of tf2mart shenanigans.

https://backpack.tf/profiles/76561198106233311#!/compare/1523232000/1523318400

https://backpack.tf/profiles/76561198116230594#!/compare/1522800000/1523664000

Doesn't match.

https://backpack.tf/profiles/76561198246187182#!/compare/1523318400/1524787200

https://backpack.tf/profiles/76561198055694560#!/compare/1523750400/1523836800

SCM.

https://backpack.tf/profiles/76561198356119069#!/compare/1522368000/1522454400

Unusualified.

https://backpack.tf/profiles/76561197996229893#!/compare/1524787200/1524960000

https://backpack.tf/profiles/76561198069393071#!/compare/1524960000/1525046400

Untrace.

https://backpack.tf/profiles/76561198073960192#!/compare/1520726400/1523577600

More untrace SCM stuff. Sold a bunch in that period.

https://backpack.tf/profiles/76561198334637469#!/compare/1521763200/1523145600

https://backpack.tf/profiles/76561198289065294#!/compare/1522281600/1523664000

Bulk. The Honcho was involved as well.

https://backpack.tf/profiles/76561198400492976#!/compare/1523145600/1523232000

Unusualified.

https://backpack.tf/profiles/76561198114402400#!/compare/1522800000/1522886400

https://backpack.tf/profiles/76561198161215105#!/compare/1522800000/1522886400

Sold for an unusual SMG + The Head Full of Hot Air. This seems rather high, and likely would've been B/O capped.

https://backpack.tf/profiles/76561198161215105#!/compare/1523318400/1523577600

https://backpack.tf/profiles/76561198114545227#!/compare/1523232000/1523318400

Better used on the High Five. Sellers at 19-20 there so supports range.

https://backpack.tf/profiles/76561198142082556#!/compare/1525651200/1525996800

https://backpack.tf/profiles/76561198121913969#!/compare/1525910400/1525996800

Sold for 18 keys.

https://backpack.tf/profiles/76561198388607790#!/compare/1524182400/1524355200

https://backpack.tf/profiles/76561198057527045#!/compare/1524096000/1524182400

Unusualified.

https://backpack.tf/profiles/76561198114545227#!/compare/1522454400/1522540800

https://backpack.tf/profiles/76561198025083656#!/compare/1522368000/1522540800

Counted all keys, 23 keys match.

https://marketplace.tf/items/30672;5;u3005

Only recent sale at $39.21 ( / $1,98 ) = 20 keys. The earlier marketplace.tf sale at $60 is too high and the other one at $39.22 supports range.

https://gyazo.com/93970900575bda2ef5ba031761533031

https://imgur.com/a/GGNBBpj (my sale)

My sale at 25 keys.

Sales: ±18 / 18 / 18 / 19 / 19 / 20 / 22 / 23 / 24 / 25 keys

Sellers: 25 / 30 / 30 / 200 keys

https://gyazo.com/bea620440e2c791fd8d4441d8b5d0a55

There are no clear indications that either one of the ends are outliers. I've only used sales in the past 2 months. 18 is a clear common trading point (not countered either by buy orders), supported by sales at 19 and 20 keys. 25 keys doesn't seem high either, it was full pure and sales at 23 / 24 to back it up. Also 2 sales that support this range in general. Probably best to include all sales in the range. Plenty of sales at around 20-22 to support a range.

I'd go with 18-25 keys here.