The ISSN International Centre and the Spanish ISSN National Centre have recently completed a common project of semi-automated retrospective assignment of ISSN to bibliographic records from the Spanish Serials Union Catalogue (Catálogo Colectivo de Publicaciones Periódicas), which is managed at the National Library of Spain (which also hosts the ISSN National Centre).
More than 10,737 ISSN records were thus added to the ISSN Register. The records initially only had an internal identification number (ISXN), structurally similar to the ISSN but with no external recognition. Assigning retrospectively ISSN numbers to these records allowed for a confirmed international identification of the publications concerned. Thanks to this process, the coverage of the ISSN Register as an authoritative international reference source was also improved.
More details about the different steps of the project can be found in the attached summary report (in Spanish) produced by the Spanish ISSN National Centre.