Bava Metzia 21 - Lost Objects that the Finder May Keep
When an object without an identifying mark is lost, the owner despairs of recovering it, because he knows that even if it is found, he will be unable to prove that it is his. Therefore the following objects belong to the finder: scattered produce, scattered coins, small sheaves in a public domain, round cakes of pressed figs, baker's loaves.
When someone loses such an object but before he discovers his loss - Rava says that the object is already abandoned, but Abaye disagrees. The law in this case follows Abaye: until the owner has consciously despaired of recovering the lost object, the finder is not permitted to keep it.
This is the "Y" in the six cases abbreviated Y A L K G M where the law follows Abaye against Rava.