After a bit of googling, I found that it should read: Cum Caesarea Maiest. Galliarum Regis, ac Senatus Veneti gratia & privilegio, ut in diplomatis eorundem continetur.
My wild guess is in this direction: "The emperor, king of the Gallians (The French), and through the grace and law of the Venetian Senate, in order to maintain their privileges."
If someone's Latin is better, this is the context, clearly it is some kind of copyright phrase but unfortunately there is no literal translation available:
In 1540, Robert Winter, according to the title-page of his edition of Homer's Iliad, had even been awarded a ten-year privilege: "Cum gratia & priuilegio tam Regio quam Caesareo ad decennium".[37] Fourteen book titles out of the sample of 100 refer to both Imperial and French privileges: two of these (1543, 1571)[38] do not indicate a term, two (1536, 1546)[39] refer to a five-year term, one (1546)[40] to a seven-year term, and six to a ten-year term of protection.[41] Three of these title-pages indicate a different term of protection for each of the two ?countries' concerned: five years in the Empire and ten in France in one case,[42] ten in the Empire and six in France in another,[43] and, finally, ten in the Empire and nine in France.[44] One title refers to privileges from the Emperor, the King of France, and the Venetian Senate: "Cum Caesarea Maiest. Galliarum Regis, ac Senatus Veneti gratia & privilegio, ut in diplomatis eorundem continetur."[45]