The S and Z symbolize Spiritus Sanctus for being the first latin letter of Spiritus.
It is third person of the Trinity.
The S is a sign that appears by first time in Spain in the Chrismon of Jaca Cathedral converting the Christological Chrismon in the Trinitarian: P is Pater, X Filius and S Spiritus. The Chrismon of Jaca Cathedral has on his ring a latin inscription that reads: "In this sculpture recognizes, reader, the following: P designates the Father, A the begotten, and double (letter) the Spirit begetter, the three are indeed one and the same Lord".
Notre Dame d’Orcival (Francia), Santa María de Veruela (Zaragoza), San Martín de Frómista (Palencia), Santa María de Eunate (Navarra), Iglesia de San Frutos del Duratón (Segovia), San Bartolomé de Ucero (Soria), San Pedro de las Dueñas (León), San Juan Bautista de Castiliscar (Zaragoza).
Read more in The stonemason’s signs in the Romanesque churches in Spain.