The two digit number stamped on the seat tube lug is not a serial number. Rather, the Condorio site explains it this way...

"The two numbers stamped on the back or side of the seat lug on Legnano frames after 1966 were not serial numbers and by no means should they be confused with ‘team bikes’ or representative of the year the frame was produced or the size of the frame. They also do not reference the Legnano model as I have found the same two digits on both city bikes and road bikes alike.
After many years of looking for an explanation, I have learned that these numbers were referenced to the specific ‘production unit’ within the Bozzi Spa factory that produced the frame. In other words a tracking reference for QA purposes and perhaps also to monitor the overall productivity of the production unit."

Based on that information, I came to believe that the vintage of my bike was 1968. To this day, I am not sure of the vintage, only the make (Legnano) and model (Gran Premio). But I am sure that it is my favorite bike (I own four vintage road bikes these days, not counting two project bikes that are just waiting).