That is most likely not the only sale in all of these item histories. You should try looking at them.

https://backpack.tf/item/5221268321

https://backpack.tf/item/5242213385

https://backpack.tf/item/5245243740

https://backpack.tf/item/5247297179

https://backpack.tf/item/5255042505

And if you need a guide on how to find sales, check this out.

https://forums.backpack.tf/index.php?/topic/59832-guide-how-to-use-the-compare-function/

Good luck!

The Larrikin involved in your sale is outdated and needs a mini: https://backpack.tf/profiles/76561198081937913#!/compare/1506556800/1506643200

Larrikin mini: https://backpack.tf/profiles/76561198108148076#!/compare/1508112000/1508198400 - 11 pure

+

https://backpack.tf/profiles/76561198081937913#!/compare/1506384000/1506470400 - Stormy Milkman (mature sellers @ 13, taking that) + negligible sweets, rounding to 13

So Larrikin is ~12 currently, and the sweets in that trade are closer to ~3 keys, meaning the trade should be valued ~15 keys.

Additional sales:

https://backpack.tf/profiles/76561198394877897#!/compare/1507334400/1507420800 - 15 pure

https://backpack.tf/profiles/76561198192602363#!/compare/1509148800/1509235200 - 14 pure

Multiple recent sales for less further support that the flat value here is going to be high.