The reference is to Notebook N (Notebook N p.29) which begins in Amiens but is continued on this journey. Ruskin commented in Notebook N (Notebook N.Front free endpaper recto) that it marks the beginning of his thinking about Stones of Venice. The fact that Ruskin gives only the page number of the notebook here suggests that this was the only small notebook he was using at this stage of the trip. Later references to other notebooks specify which notebook he has in mind.
[Version 0.05: May 2008]