Couple more sales in that history:

https://backpack.tf/item/1247257217

https://backpack.tf/profiles/76561198140316312#!/compare/1613433600/1613520000

https://backpack.tf/profiles/76561198049469629#!/compare/1613433600/1613520000

Seems to have sold for:

160 keys

Strange Festivized Australium Grenade Launcher

Vivid Plasma Killer Exclusive

Sale is ~250 keys or so

https://backpack.tf/profiles/76561198049469629#!/compare/1613952000/1614038400

https://backpack.tf/profiles/76561198114402400#!/compare/1613952000/1614038400

This one is a bit weirder.

Purple Energy Honcho's Headgear

Scorching Flames Brown Bomber

These 2 match but that's only around ~200 in unusuals, I imagine more was involved but then used again in other trades really quickly

It'll need some confirmation

That's about it, if the 2nd sale was actually ~200 in unus, well then it's 250 >> 200 >> 300 so I guess 200 - 300 would work, that's just rough estimates, wait for a mod to comment I suppose but I imagine working out minis and maybe getting in contact with the people involved in the second sale just to confirm is the next move.

Best of luck, congrats on the buy