I'm sure Dumbledore got the memories about Tom's childhood and young adulthood a long time before 1981 (except for the one from Slughorn). I can't imagine him not investigating anything all those decades that Tom/Voldemort was raising trouble, nor can I imagine Morfin Gaunt surviving in Azkaban for 40 years. But the fact that Dumbledore doesn't look for Tom's Horcrux (he didn't expect there to have been more than one initially) shows that until Tom proved to be immune to a rebounding AK Albus didn't think he had one (after all, neither did Gellert, so I guess Horcrux making isn't a requirement of Wizarding Evil Overlords).
After Halloween 1981 Albus must have known Voldemort had a Horcrux - it is the only way one can survive an AK that connects. It is entirely possible he realized very shortly that the Horcrux was Harry and that his placement in the Muggle world was related to that bit of information - whether because he wanted to keep the Horcrux from detection, or keep it under observation to see that Harry doesn't become mini-evil-overlord before he turns 11. The fact is that after COS when Albus supposedly realized the diary was a dispensable Horcrux he still did not search the back-up. So I think by then he was convinced Harry was the other Horcrux.
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timeasmymeasure
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Date: 2010-09-14 09:00 pm (UTC)After Halloween 1981 Albus must have known Voldemort had a Horcrux - it is the only way one can survive an AK that connects. It is entirely possible he realized very shortly that the Horcrux was Harry and that his placement in the Muggle world was related to that bit of information - whether because he wanted to keep the Horcrux from detection, or keep it under observation to see that Harry doesn't become mini-evil-overlord before he turns 11. The fact is that after COS when Albus supposedly realized the diary was a dispensable Horcrux he still did not search the back-up. So I think by then he was convinced Harry was the other Horcrux.