The Sociedad del Timbre (1874 - 1879) was responsible for the printing and distribution of documents with fiscal seals. To authenticate the documents, the seals shown in the post were stuck to the sheets. They served as a countersignature. Without the seal, the document was not valid. The seals carried the name of the province (most Spanish provinces have the same name as the provincial capital) where the document was expedited.
The discurso académico XVIII of the Real Academia Hispánica de Filatelia by Joaquín Amado Moya has more information.
