I wonder, annually, about the Mass proper for S Josaphat, whom we 'kept' quite recently.
The Collect begins "Excita ..." The Introit began "Gaudeamus ...".
These are both first millennium formulae common in the Roman Rite. It is almost as if the nineteenth century popes or functionaries who were responsible for this Mass wanted to say "The Unia which existed in the first Millennium between East and West, for which S Josaphat was martyred, makes it suitable that, when we Latins celebrate this Saint, we should do so with formulae redolent of our own liturgical Tradition during that First Millennium".
He was beatified in 1643 by Urban VIII (1623-1644). The ODCC says that he was the first saint of the Eastern Church to be formally canonised after process in the Congregation of Rites, in 1867, that is, during the pontificate of Pius IX. Later, Leo XIII made his observance universal. But he had been beatified in 1643, and the usual process was for a candidate to be beatified and then assigned propers printed in the Appendix pro aliquibus locis. If that candidate was subsequently canonised, and if a future Roman Pontiff desired to make his cultus universal in the Latin Churches, the liturgical provision already available in the the Appendix would often be yanked out and made the liturgical provision of the whole (Latin) Church.
Incidentally, collects beginning Excita can be found in earlier Latin Sacramentaries: the Verona Sacramentary has such a collect for S Lawrence in August; 'Gelasianum' one in Holy Week.