Please see the most recent update here on the two-digit production numbers:

The two numbers stamped on the back or right side of the seat lug on Legnano frames produced 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, these two numbers were a tracking reference for QA purposes and perhaps also to monitor the overall productivity of the frame making unit.

Above is an example of the two digit production code on the right side of a 1969-1970 Gran Premio model (horizontally on the back of the seat lug for Roma Olimpiade models only). Based  on my research, these two digit numeric production codes were used for only a few short years between 1967 and the early 1970’s.