Perhaps you should try some spelling variations:
"serie letras de clase." » "serie las de clase.", "serie letras de clacso.", "serie letras la clase."
"serie detras de base." » "serie lecturas de base.", "serie retratos de base.", "rein detras de base.", "serie detras de bases."
"serie actas de clases." » "serie fichas de clases.", "serie actas de bases.", "serie actas del clases.", "serie cartas de clases.", "serie actas de clacsos.", "serie actas la clases."

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (((("serie actas de clases.") or ("serie letras de clase."))) or ("serie detras de base."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("serie actas de clases.") OR ("serie letras de clase."))) OR ("serie detras de base."))*.
• Removing quotes may allow a broader search: (((( serie actas de clases. ) or ( serie letras de clase. ))) or ( serie detras de base. ))*.