The first print edition of Plato in Greek was done by Henri Estienne (in Latin, Henricus Stephanus). Accordingly, all of Plato is cited according to the page number a passage had in the original Stephanus edition, with a letter between a and e to indicate how far down the page the passage is: Stephanus pagination – Wikipedia
So in your example, St. II p. 70 means page 70 of Vol II of the Plato edition by Stephanus starts at that point. The usual citation would drop the volume number, and just give the Plato work, the page number, and a letter. For example, Alc II 140c, meaning halfway down Vol. II p. 140 in Stephanus’ edition of Alcibiades. You just cite Meno as Meno 70a, 70b, 70c, 70d, or 70e, depending on how far down the page you are; we already know Meno is in Vol. II. The letters A-E should appear in the sidebar.