Credits to Bentry & Shuffle Spy for collecting sales on this.

Just posting it cause its still unpriced.

(Credits to Bentry)

https://backpack.tf/profiles/76561198093300204#!/compare/1647388800/1701734400

https://backpack.tf/profiles/76561198928388508#!/compare/1701216000/1702080000

?

https://backpack.tf/profiles/76561198317171754#!/compare/1696723200/1696896000

https://backpack.tf/profiles/76561198063402284#!/compare/1696723200/1696896000

?

https://backpack.tf/profiles/76561198988396980#!/compare/1697932800/1698192000

https://backpack.tf/profiles/76561198075267569#!/compare/1702425600/1702425600

?

https://backpack.tf/profiles/76561198126133559#!/compare/1700524800/1700611200

https://backpack.tf/profiles/76561198031246215#!/compare/1651708800/1701648000

?

https://backpack.tf/profiles/76561199107865875#!/compare/1698710400/1698796800

https://backpack.tf/profiles/76561199078552843#!/compare/1698710400/1699056000

1 key

https://backpack.tf/profiles/76561198311228706#!/compare/1702166400/1702252800

https://backpack.tf/profiles/76561198124054313#!/compare/1667952000/1702857600

?

https://backpack.tf/profiles/76561199232380670#!/compare/1699142400/1699228800

https://backpack.tf/profiles/76561198360351051#!/compare/1698624000/1699142400

?

https://backpack.tf/profiles/76561198000518945#!/compare/1697932800/1698019200

https://backpack.tf/profiles/76561199502156612#!/compare/1697932800/1698019200

here

https://backpack.tf/profiles/76561199502156612#!/compare/1698019200/1698710400

https://backpack.tf/profiles/76561198802360768#!/compare/1697500800/1698019200

bulk to Searing Plasma Hat Outta Hell

https://backpack.tf/profiles/76561198802360768#!/compare/1698019200/1698192000

https://backpack.tf/profiles/76561198132961995#!/compare/1698105600/1698192000

+20 keys to Fireflies Flame Warrior

https://backpack.tf/profiles/76561198132961995#!/compare/1698364800/1698537600

https://backpack.tf/profiles/76561198058740774#!/compare/1698019200/1698537600

18 keys

https://backpack.tf/profiles/76561198068986329#!/compare/1703721600/1703808000

https://backpack.tf/profiles/76561198909167310#!/compare/1703721600/1703808000

show 9 keys

https://backpack.tf/profiles/76561198125290682#!/compare/1703635200/1703721600

https://backpack.tf/profiles/76561198068986329#!/compare/1703635200/1703721600

bulk to Scorching Flames Prince Tavish's Crown

https://backpack.tf/profiles/76561199226606088#!/compare/1703721600/1703808000

https://backpack.tf/profiles/76561198125290682#!/compare/1703721600/1703808000

Strange Professional Killstreak Australium Force-A-Nature

Past 7 days

6 @ 23 keys 15 ref

1 @ 24 keys 10 ref

23 keys

https://backpack.tf/profiles/76561198859876939#!/compare/1703635200/1703721600

https://backpack.tf/profiles/76561199226606088#!/compare/1703462400/1703635200

19 keys

https://backpack.tf/profiles/76561199044886703#!/compare/1703376000/1703462400

https://backpack.tf/profiles/76561198859876939#!/compare/1703203200/1703462400

show 2 keys

More sales: (Credits to Shuffle Spy)

https://backpack.tf/profiles/76561199115060021#!/compare/1697068800/1697328000 - ?

https://backpack.tf/profiles/76561198054270952#!/compare/1697241600/1697414400 - Bulk for something

https://backpack.tf/profiles/76561198054270952#!/compare/1697760000/1697846400 - 3 Keys show

https://gladiator.tf/sales?item=Fireflies%20Brothers%20in%20Blues&at=2023-10-21T23:47:26.352Z - It should be one of these 17 Key sales

https://backpack.tf/profiles/76561198166035040#!/compare/1649721600/1697673600 - ?

https://backpack.tf/profiles/76561199232380670#!/compare/1697846400/1697932800 - 16 Keys (see imgur album)

https://backpack.tf/profiles/76561199232380670#!/compare/1699056000/1699142400 - 16 Keys (see imgur album)

https://imgur.com/a/2D91kwa

https://gladiator.tf/sales?item=Fireflies%20Brothers%20in%20Blues&at=2023-12-31T23:47:26.352Z - A more recent 17 Key sale

https://backpack.tf/profiles/76561198199244659#!/compare/1703030400/1703116800 - Bulk sale

https://backpack.tf/profiles/76561198008840775#!/compare/1702771200/1703116800 - Don't know how many Keys were for the Brothers

https://backpack.tf/profiles/76561198008840775#!/compare/1703376000/1703894400 - 17 Keys + ~1 Key of items

https://backpack.tf/profiles/76561199037684057#!/compare/1700524800/1700611200 - IDs match the 19 Keys

https://backpack.tf/profiles/76561198376624864#!/compare/1701648000/1702080000 - ?

https://backpack.tf/profiles/76561198102396832#!/compare/1702080000/1702166400 - Bulk sale

https://backpack.tf/profiles/76561198102396832#!/compare/1702339200/1702425600 - 3 Keys appear

https://backpack.tf/profiles/76561199132184792#!/compare/1702339200/1702425600 - ?

https://backpack.tf/profiles/76561198163289648#!/compare/1702080000/1702252800 - Bulk sale

https://backpack.tf/profiles/76561199110809353#!/compare/1696032000/1696118400 - IDs match with 15 Keys (low)

https://backpack.tf/profiles/76561199202208101#!/compare/1697760000/1697846400 - 40 Keys?

https://backpack.tf/profiles/76561198086011812#!/compare/1697846400/1697932800 - Bulk sale

https://backpack.tf/profiles/76561198086011812#!/compare/1698883200/1698969600 - ?

https://backpack.tf/profiles/76561198141685007#!/compare/1701043200/1701129600 - ?

https://gladiator.tf/sales?item=Fireflies%20Brothers%20in%20Blues&at=2023-11-02T02:12:08.876Z - Unknown potential sale at 22

https://marketplace.tf/items/tf2/31356;5;u291 - 3 sales at $59.99 (34, 35, and 35 Keys)

https://gladiator.tf/sales?item=Strange%20Professional%20Killstreak%20Australium%20Force-A-Nature&at=2024-01-01T02:52:06.470Z - You forgot this

The "9 Key" sale was 9 Keys + the Cloud 9 Tam O'Shanter which quicksold for the 17 Keys + 1 Key of Refined you see in the seller's compare.

That makes all of the Brothers in Blues sales 15, 16 x2, 17 x7, 18 x3, 19 x2, 22 (maybe), 23, 34, 35, 35, 40 (?). 18 - 23 would still work.

Seller @ 30 (19 days old)

Buyer @ 18

https://imgur.com/a/nCNxUiB

Taking 18-23 keys