The link to the software for Spanish plates is apparently back (it was dead a few weeks ago), but certainly better than a limited software asking for MS Windows to be used, Exiv96 gave us the link to a website with a lot of information:
http://www.sme-matriculas.es/
The 7th item (Datación de matrículas. Sistema actual y anteriores // Tables for plate dating for current and previous numbering system) is certainly the most interesting for us.