In my beautiful, red leather, Missale Romanum (e Typographia Haniquiana), 1840, last Saturday's festival of the Expectation of the Childbearing of the Blessed Virgin Mary is marked in the Missae Sanctorum celebrandae aliquibus in locis ex indulto Apostolico as "Pro omnibus Hispaniarum Regi subjectis" ("For all the subjects of the King of the Spains").
Am I right in suspecting that the idea of using plurals when referring to a King of a world-wide empire began with His Most Catholic Majesty? That Third Rome then cribbed it ("Autocrat of all the Russias")? And that after dear Dizzy had proclaimed his lady friend Empress of India, the people in charge of the inscriptions on our coins adopted it: "Britanniarum omnium Regina Indiae Imperatrix" ("Queen of all the Britains, Empress of India"*)? Or had the Hanoverians used it previously? Are there other examples?
*I think I am right in recalling that the Director General of the British Museum, in his recent "History of the World in 100 Objects" series, translated it as " ... of all Britain". There's Art Historians for you. Show them the simplest piece of Latin and you can infallibly rely upon them to mistranslate it.