'Two & A Half Men' Billboard -- Ashton Unravels

It's barely been a month since Charlie Sheen's face adorned the outer wall of Warner Bros. studios -- but today, Ashton Kutcher has finally entered the picture ... with significantly less clothing.

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The generic billboard -- which replaced Charlie's old one in June -- went down an hour ago ... and in its place ... a brand new poster with the entire new cast.

Charlie who?