Roughly a dozen Early Cyrillic letters have no Greek model. The majority reading derives them from Glagolitic, which is older and already fitted to Slavonic. A second reading compares sha, tsy and zhivete with Hebrew or Samaritan letters, an argument made since the nineteenth century. A third holds that shapes this simple cannot support either claim. No exemplar of either alphabet survives from the ninth century, when the choices were made, so every argument runs on shape comparison against later manuscripts.
What would resolve this A dated exemplar of either alphabet from the period of composition, or a decisive argument from the internal structure of the two letter sets rather than from resemblance between individual shapes. Failing either, a rigorous statistical treatment of shape similarity that establishes what level of resemblance would count as evidence at all.
OpenSlavic palaeography, with Semitic epigraphy for the comparative claimsFrancis Dvornik, Byzantine Missions Among the Slavs: SS. Constantine-Cyril and Methodius