Freek-
Details such as the seat stays ends at the seat lug, seat lug cut outs and fork crown appear very similar to a GC on the Speedbicycles.ch site reperesened as a 1977 build. The holes on the drive side dropout point to 1978, as the #17a Campagnolo catalog shows the chain rest.
Dates are approximate of course. The 30 on the BB shell and the steerer is associated with a respray at the Vigorelli. Kind of a tracking number.
As the Speecbicycles site does not claim original paint for the '77 bike, and if so, usually states the transfer set could be off. I think the Alberto signature is correct for this bike, small possibility it is an Prestige, the seat stay caps would guide me away though.
The top tube cable routing is a later modification, I guess an M on the BB shell would be a loud "not a Gazelle" feature, I would think the BB shell armor could be removed.
Must have been a strong need for it to be put incognito.
John Jorgensen
Torrance Ca USA