Marble slab with Chi-Rho monogram and Alpha and Omega in a circle; traditionally associated with St. Ambrose (4th century), and possibly indeed of this period. A frame with inscriptions was added in the baroque period (supposedly in 1669, when it was moved to a "baroque corner" of the refurbished dome after it had been kept in storage since 1577). The inscription above reads Chrismon Sancti Ambrosii (i.e. "oracle of St. Ambrose", whence the name chrismon for the Chi-Rho monogram), the one below gives an interpretation of the symbol,

Circulus hic Summi continet nomina Regis / Quem sine principio et sine fine vides / Principium cum fine tibi denotat Α Ω

"The circle here contains the name of the Highest King, whom you see without beginning or end; Alpha and Omega signify the beginning and end for you"

Literature:

• "1669, 6 settembre: E' sistemato nel coro del Duomo, dov'è ancora oggi, il Chrismon di S. Ambrogio che si trovava nei magazzini della Fabbrica dal 1577. Altri chrismon a Milano si trovavano a S. Ambrogio, S. Giorgio in Palazzo e S. Martino in Compito." (storiadimilano.it)
• A. L. Millin, Voyage dans le Milanais (1817), p. 51 (trans. C. L. Ring, Mailand und seine Umgebungen, 1825, p. 72).
• Kenelm Henry Digby, Mores Catholici, Or, Ages of Faith vol. 1 (1844), p. 300.