F symbolizes Christ-son, because it’s the first latin letter of Filius, second person of the Trinity.
Churches:
Santa María de Moreruela (Zamora), Santa María de Carracedo (León), San Isidoro de León, Santa María del Camino de León, Catedral de Santiago de Compostela, Iglesia de Vilar de Donas (Lugo), San Juan Bautista de Castiliscar (Zaragoza).
Read more in The stonemason’s signs in the Romanesque churches in Spain.