Leica WATE 16-18-21mm f/4 Tri-Elmar
This legendary lens was the first lens incorporating three separate focal lengths available for an M series Leica. It is a magnificent piece of optical engineering, the super-complicated cutaway of the lens can be seen here:

Also here is a through review by Steve Huff:

I was lucky to have this lens and shoot with it, very impressed at its performance and apperciate the marvellous build quality, truly a lens to admire and enjoy.
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