This, is, going to be weird. No, seriously.

Ok, uh, prepare yourself...

Only Sale

http://www.tf2outpost.com/trade/25941417 Sold for what appears to only be 19 keys using Julia's script, however, I would presume it is 23. (reasoning below)

B/o for Nickel was 23

X = 23

Keys http://gyazo.com/b1348db530625ba5f5b4a41a2047e482

Noble https://backpack.tf/item/1712703640

Reasoning

Well, this will be a wall of text, so, bare with me.

We see the 19 keys in his backpack. However that bud as well? The name Robo-Phob doesn't appear in the history. I have checked each previous owner of the bud to find 0 results of keys from MakinDemStaks. I would assume since the difference in the 2 bp histories is 2 months apart, leading to inaccuracies, that he sold those keys for that bud when they were 4 keys, or transferred them probably somewhere else. The rest of the keys however have Robo-Phob in the name. On his OP trade, http://www.tf2outpost.com/trade/25941417 Robo-Phob claimed to have sent the offer. The item ID of the bud as well is more than 2400 and is in fact 71035 difference. This would also give evidence to the fact keys must've been used to buy the bud in the time the backpacks were both scanned. The B/O was also 23 keys, and buds were 3.5-4 keys.

so, i would call it safe to presume 4 keys were used to buy the bud, and they do not show up in the bp comparisons.

Ciao~