Hello,
I see that this bike is listed in the serial number listing on my CONDORINO
website along with a single photo of the bike fully built. From your post it does not say if you purchased the complete bike or just the frame?
Based on the 1952 Bozzi parts catalog, I do not see the headset shown in your photos listed among the available models from Gnutti, Way-Assauto, Magistroni or Maiocchi. And the headset is not familiar to the Gran Premio model from my own collecting and research, leaving me to believe that it is not original to the bike and may explain why you do not have the top nut.
The Legnano Roma headseats from that era were either from Campagnolo (first Gran Sport model) or Magistroni. The Gran Premio models from these years were also fitted with the Magistroni headset or the almost identical looking headset from Way-Assauto. Depending on your plans for the bike, I would replace the current fittings with either of these headsets if you are looking for a period correct result.
best regards,
Mark Campbell