6-speeds were spec'd on 1972 Raleigh Pro's. Super Record derailleurs were in use by 1975. 1974 photo's show Merckx on NR components.

I can't get the direct link, but on veloaficionado.com there are pics of Merckx inducted into the Giro d'Italia Hall of Fame. This bike may have been authenticated by Merckx at that event. I can't get the direct link cause i pulled it up on my iphone. Same track pedals, chainring. Same bike as pictured in post #33 .