Originally, the three heads, modeled by Philip Martiny, were all of Minerva, which the architects found unsatisfactory. Therefore, in 1909 the sculptor François Tonetti-Dozzi was commissioned to transform two of them into the heads of Juno and Mercury (1986 Reed, Morone and Norton, p. 65)
Bibliography
1986 Reed, Morone and Norton Henry Hope Reed, Francis Morrone. W.W. Norton, The New York Public Library, The Architecture and Decoration of the Stephen A. Schwarzman Building, London, 1986, p. 65