The headword 'tabellio' is a Latin word for a notary or drafter of legal documents: see OLD s.v. tabellio, quoting Ulpian Digest 48.19.9.4. Synonymous is 'tabellanio' (Edict of Diocletian 7.41; cf. Souter s.v.).
[1] This word for lawyer (nomikos) is used in the New Testament, at Matthew 22:35 and elsewhere, as a general word for the (Jewish) lawyers who questioned Jesus. Its New Testament use would mean that the word was familiar in the compiler's Byzantine times, which I take to be the implicit background to 'called popularly'.
[2] Procopius, Secret History 28.6; see in context at pi 2303.
Though the final phrase here might mean 'with his own letters', I take it to refer to use of a seal to make an impression; if this is correct, 'grammasi' would refer to the seal design, not to letters of the alphabet.