This historiated initial ‘L’ appears at the beginning of the edition of Livy’s Decades published by Johann Herwagen in Basel in September 1555. While depictions of playful putti were an established convention of historiated initials, what are we to make of this pair? Well, it turns out that they have been noticed before, appearing for the first time in the 1543 Basel edition of the Fabrica of Vesalius. They dwell there not in isolation but as part of a twenty-three letter suite depicting putti behaving like medical students — vivisecting a pig, snatching corpses for anatomical study, etc. The website of Karger Publishers advertising their newly published English translation of the Fabrica provides detailed illustrations of the suite: http://www.vesaliusfabrica.com/en/original-fabrica/the-art-of-the-fabrica/historiated-capitals.html. Moreover, Karger lists several studies, including one by Dr. Samuel W. Lambert (1859-1942) available in Hathi Trust. A relevant excerpt follows:

Samuel W. Lambert, ‘The initial letters of the anatomical treatise, De Humani Corporis Fabrica, of Vesalius’, in T. A. Malloch (ed.), Three Vesalian Essays to accompany the Icones Anatomicae of 1934 (New York: Macmillan, 1952), pp. 1-24).