Perhaps they are Indalo symbols Stephen. Or perhaps a simple crossbow. Whatever they represent they still would have been added by medieval craftsmen who constructed the cathedral! The multiplicity of such symbols is vast.
For a scholarly account (and superb bibliography) describing such marks in León and Santiago de Compostela see Therese Martin, Reading the Walls: Masons’ Marks and the Archaeology of Architecture at San Isidoro, León, in Church, State, Vellum, and Stone: Essays on Medieval Spain in Honor of John Williams, eds. T. Martin and J. A. Harris, Leiden, 2005. (Readable on-line).