What do you mean by "source wallet address"?
kjj ... I did see your post in the other thread ... SatoshiDice and the like can auto return funds, so there must be a way
Just as a reminder, while this like it should work, the concept of a sending address does not exist in bitcoin. What this is doing is finding the last address to have received these funds. This is occasionally what you expect, but not always. If you are thinking that this address is owned or controlled by the person that sent you a transaction, you will be wrong fairly often.
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