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https://books.google.com/books?id=_esuAAAAMAAJ&lpg=PA104&ot...
The Royal Tribes of Wales By Philip Yorke page 84 gives Rhodri "Molwynog" as ancestor of George lll:
George the Third, the eldest son, by Augusta of Saxgotha, of Frederick, Prince of Wales, the son of George the Second, the son of George the First, the son of Ernest Augustus, Elector of Hanover, by Sophia, the daughter of Frederick Elector Palatine, and Elizabeth, the daughter of James the First, the son of Lord Darnley, and Mary, Queen of Scotland, the daughter of James the Fifth, the son of James the Fourth by Margaret, the eldest daughter of Henry the Seventh by Elizabeth, the eldest daughter of Edward the Fourth, the eldest son of Richard Duke of York, the son of Richard of Conisburg Earl of Cambridge, by Anne daughter and heiress of Roger Earl of Marche, the son of Edmund, Earl of Marche, by Philippa daughter and sole heiress of Lionel Duke of Clarence, the third son of Edward the Third. This Edmund was the son of Edmund Mortimer, the son of Roger, the first Earl of Marche of this family, the son of Edmund, the son of Roger, the son of Ralph by Gwladys Ddu, or the Black, the heiress of her brother Dafydd ab Llywelyn, the son of Llywelyn ab lorwerth, or Leolinus Magnus, Prince of North Wales, the eldest son of lorwerth drwyn-dwnn, the eldest son of Owain Gwynedd, the son of Gruffudd ab Cynan, the son of Cynan, the son of lago or James, the son of Idwal, the son of Meurig, the son of Idwal foel, the son of Anarawd, the eldest son of Rhodri fawr, or Roderick the Great, the son of Merfyn frych, and Esyllt, the daughter and heiress of the last Prince Cynan Tindaethwy, the son of Rhodri Molwynog, the son of Idwal iwrch (or the roe) the son of Cadwaladr, the last King of the Britons, who abdicated, and died at Rome in 688. His present gracious Majesty is right heir, in lineal succession, to the British, Cambro-British, Anglo-Saxon, Anglo-Norman, English and Scottish Kings.