
In a letter to Emperor Charles V, from Venice, 5 Oct, 1544; copied in the 'Archives of Simancas' by Mr. Bergenroth; as quoted by J.A.Y. Crowe & G.B. Cavalcaselle in Titian his life and times - With some account... Volume II, publisher John Murray, London, 1877, p. 103
This letter is written by Titian himself - free from the polite style of his secretary/friend Arentino; he is telling the Emperor that he had finished two portraits of the Empress Isabella, he painted after her death after a probably Flemish original. The two portraits were sent to the court in Brussels.
1541-1576
Source: https://en.wikipedia.org/wiki/Titian#/media/File:Isabella_of_Portugal_by_Titian.jpg