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I don't know, somehow that seems a little existential for mid-60s DC. I'd guess it was to be the Joker.


The Joker doesn't fit for me at all. I can't see The Joker taunting Batman anonymously (and Batman not recognizing him). The superscience of the Outsider is also well beyond anything the Joker ever did. Not to mention, knowing all of Batman's secrets, like the location of the Batcave. (And the distorted image in Detective #340 has some of the angular features of Batman's cowl.)

Assuming it wasn't a cheat and wasn't someone entirely new, a really advanced scientist that knows the location of the Batcave doesn't leave much for suspects. Dr. Destiny, Amos Fortune, or someone of that ilk probably is a better candidate than The Joker.