New letters and a call for assistance

4 unrecorded letters between Antonio Agustín and Jerónimo Zurita (most likely from the Zurita manuscript correspondence at the Real Academia de la Historia) have just been added to the database. Although published since the 18th century in vol. 7 of Agustín's "Opera Omnia", they seem to have escaped the attention of modern scholars who have catalogued Agustín's correspondence.

Even though all four lack any mention of the year, I have been able to date two of them through reference to the death of well-known erudites : Ottavio Pantagato in the case of the 1568-06-01 letter, and Pedro Ponce de León, Bishop of Palencia, in the case of the 1573-01-28 letter. The specific year of the other two letters eludes me at this moment. However, both can be traced back to the first half of the 1570s, since one evokes the still recent death of Juan Páez de Castro (which occurred at the very beginning of 1570), while the other refers to secretary Antonio Gracián as having completed his catalog of the Escorial library (which he did sometime in 1575; he died the following year).

If anybody out there can figure out the exact date of these two letters, either through consultation of the original manuscript or from internal context, I would greatly appreciate the help. I will be sure to acknowledge your collaboration. Here is the link to the digitized copy of vol. 7 of Agustín's Opera Omnia (they are on page 203 and 206-207):