Help ID this chrome sofa  

It looks very much like the S412 sofa by Thonet BUT the examples I've seen don't have that centre leg. Yours may be a wider version, however, See this one, which is possibly a little bit less wide..
http://www.sebworld.de/Thonet-S-412-Sofa-/-Designersofa
I believe it was an in-house design.

I would say the center leg is not even the most obvious difference. The curve of the frame is much different. The Thonet sofa is more angular at points, while the op's sofa frame is a continuous curve.

You're absolutely correct, of course, Emcee M. And the OP's sofa has some visible fixings inserted in the frame at the side, whereas the S412 lacks these. Thonet make so many variations on a theme, though- all those tubular metal Breuer and Stam designs- that I thought it might be a slightly different version of the sofa with a slightly different model number!