“The Dovetailed Block”

Another puzzle from Henry Ernest Dudeney:

http://www.gutenberg.org/files/16713/16713-h/16713-h.htm

“Here is a curious mechanical puzzle that was given to me some years ago, but I cannot say who first invented it. It consists of two solid blocks of wood securely dovetailed together. On the other two vertical sides that are not visible the appearance is precisely the same as on those shown. How were the pieces put together?”

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