No hits in TLG for σεκουνδοκέριος, but six for σεκουνδοκήριος, evidently a church official at the Synod of Constantinople and Jerusalem in the sixth century. The instances all involve someone named Mēnas, who is described as "a most God-fearing reader and notary σεκουνδοκήριος of the Roman priesthood" (ὁ θεοσεβέστατος ἀναγνώστης καὶ νοτάριος σεκουνδοκήριος τῆς πρεσβύτιδος Ῥώμης). Here σεκουνδοκήριος appears to be a transliteration of a Latin word meaning (lit.) "of the second wax" i.e. "of the second wax tablet," i.e. "sub-scribe" or "under-secretary." Cf.
primicerius in
primicerius → πριμικήριος—
Concilia Oecumenica (ACO),
Synodus Constantinopolitana et Hierosolymitana anno 536 Tome 3, page 152.