It isn't clear if this is a copy. 19th-century sources make no mention of its being one; it is called a copy in a tourist guide of the 1985, while at the same time Carucci (1982) explicitly states that it is "still preserved" in Milan Cathedral, so that the tourist guides might be mistaken due to the early modern frame and inscription.

The frame and the inscription above and below the lastra were 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, the one below:

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

Literature:

• "On the wall beneath the great E. window is a tablet of marble, with a monogram of high antiquity, called 'Chrismon Sancti Ambrosii' and which contains the Α and Ω, together with other symbols. Some suppose it to be a Gnostic monument." John Murray, Handbook for travellers in northern Italy part I (1858), p. 161.
• "Nella terza campata (21), lastra marmorea con il «Chrismon Sancti Ambrosii», copia di una lastra antica" Milano, Touring Club Italiano (1985), p. 146.
• "[...] il 'chrismon sancti Ambrosii' che ancora si conserva nel Duomo di Milano. Inciso su un'antica lastra marmorea, [...]" Guglielmo De Angelis d'Ossat, Antichità, paleocristiano e bizantino, Medioevo (1982), p. 422.
• "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.