The Extra is actually the model that debuted in 1951, as described here:
http://www.classiclightweights.co.uk/
Based on photos, I believe there were pros who used these into the 1960's, despite the impact of the introduction of the Mafac centerpulls and the subsequent introduction of the Universal '6'1's. But it makes more sense to me to think that Universal '68's were actually in use in 1968 (if they mimicked the Campagnolo design, they would scarcely have been available for use that season) rather than believe that Merckx (and Gimondi, per photos of that '68 Giro) were using a 17-year-old design in preference to newer ones.
Ed Granger
Lancaster, Pennsylvania, USA (Resending this because it got tweaked in the archives for some reason - never happened before.) I'll also add that there are photos from earlier in the '68 season that show Eddy using the '61's. My money's on the' 68's debuting around the time of the Giro, but that's speculation (especially since my money's dwindling.)