I discovered today that Pope Benedict XIV’s encyclical titled Unambiguous is specifically and entirely about Lenten fasting, although the translation of an excerpt included in my missal suggested the message was about the general observance of Lent.
Summary: “The whole Church, spread throughout the Christian world, believes that it must include, among the main cornerstones of right doctrine, the Lenten fast. . . . With fasting, as if a mark of our militia, we are distinguished from the enemies of the Church. . . . No one is permitted to dispense without just reason and the advice of two doctors. . . . Even if there is no reason to illustrate what the very serious need is, we want you to know well how in such a situation we must first of all stick to a single meal. . . . We implore you in the Lord that those who cannot observe this penitential discipline common to all the faithful . . . do not neglect with other works of piety to atone for their sins and . . . with works of piety, the suffrage of prayers and the bestowal of alms.”
Breve Non Ambigimus (30 maggio 1741)
The translation from my missal, but I imagine the editors did not cross-reference and independently translate the original Italian.