Why was John not meant to pick the coltello when he was five? Why did it not belong to him?
I get that in 1954 Locke gave Richard his compass back, to prove he was from the future, so in 1961 when Locke is 5, Richard is trying to see if John has any knowledge of what is currently happening to his future self at the same time on The Island (or something in that vein). I also get that the jar of granules could be the ash surrounding the cabin, so is also linked to Locke.
That said, why is Alpert happy when Locke reaches for the Book Of Laws, which Locke hasn't had in his possession at all? Then disregards the knife, when Locke is a regular user of a knife?