Technically exact sale to STN is untraceable, but from last suggestion, the buy order on STN has remained unchanged at 6 keys 56.55 ref and has since bought two more Wings, so we could reasonably assume it was bought for that. At the time keys were also 52.72 ref, so we could take this sale as effectively ~7 keys.
https://backpack.tf/item/3134248931
https://backpack.tf/profiles/76561198316624014#!/compare/1756339200/1756425600
https://backpack.tf/profiles/76561199582907694#!/compare/1756339200/1756425600
https://stntrading.eu/item/tf2/Wings%20of%20Purity
Technically exact sale to STN is untraceable, but from last suggestion, the buy order on STN has remained unchanged at 6 keys 56.55 ref and has since bought two more Wings, so we could reasonably assume it was bought for that. At the time keys were also 52.72 ref, so we could take this sale as effectively ~7 keys.
https://backpack.tf/item/3172340807
https://backpack.tf/profiles/76561198088837823#!/compare/1756339200/1756425600
https://backpack.tf/profiles/76561198310454087#!/compare/1756339200/1756425600
Same case as above to STN, most likely ~7 keys
https://backpack.tf/item/3136744847
https://backpack.tf/profiles/76561197975508167#!/compare/1752883200/1752969600
https://backpack.tf/profiles/76561198164984732#!/compare/1753228800/1754611200
Older untraceable sale but from seller's side looks like it could potentially be 7 keys
Current 8.1 key unsolds seem to suggest older Marketplace sales may be a bit high so excluding those for now. Taking 7 flat.