Well, the division in syzygies is unknown, as shows the silence of the 19 readers and the silence of the scholars (except the pansophists of the xix century).
Nevertheless this division determines the numeration of the Plato's actual editions, which follows the Stephanus edition of 1578.
This classification is a creation of the French member of the reformed Church, Jean de Serres or Joannes Serranus, who translates the Plato's Dialogues after Ficinus (see: the pages [345-346 Serranus s.v.] of the text, The General Biographical Dictionary: Containing an Historical and Critical Account of the Lives … of the professor Alexander Chalmers here
Tomorrow I will post more, euprattein, arc.