I have found this form in Stephanus: εἰ γὰρ μάτην τεθείκασιν οἱ ὀνοματοθέται τἁ ὀνοματα, ἀλλ’ οὖν πάντως κατά τινα λόγον καὶ ἁρμονίαν.
It would be absurd if the name-givers had given the names haphazardly, but surely they did it according to reason and agreement.
how can we explain the use of τἐθεικα for τἐθηκα here?
An alternative spelling. Apparently, τέθηκα is thought to be the Attic spelling, but τέθεικα appears in inscriptions, beginning at the latest in the 3rd c. BCE. From LSJ:
pf. τέθηκα Att. Inscrr., IG 22.2490.7 (iv B.C.), (ἀνα-) ib.839.38, 1299.44, 1534.76, also at Delos, ib.11(2).161 A 6 (iii B.C.), etc., and in Papyri, POxy. 1087.42 (i B.C.); τέθεικα PCair.Zen. 324 (iii B.C.), (ὑπο-) PPetr. 3p.53 (iii B.C.), (ἐκ-) UPZ 62.4 (ii B.C.), (ἀνα-) IG 22.1011.71,80 (ii B.C.), (προσ-) Str. 1.2.23; hence some editors restore τέθηκα for τέθεικα in Attic authors, as X. Mem. 4.4.19, D. 20.55, 22.16, 27.36, Alex. 15.13; Phocian 3pl. ἀνα-τεθέκαντι BCH 59.202 (Daulis):
The sentence goes “For if the name-givers have given the names ματην, at all events they’ve given them in accordance with a certain logos and harmonia.” ἀλλ’ οὖν πάντως kicks off the main clause, “well, at all events ….”
The structure of the sentence is the same as at Pl. Phaedo 91b, εἰ δὲ μηδέν ἐστι τελευτήσαντι, ἀλλ᾽ οὖν τοῦτόν γε τὸν χρόνον αὐτὸν τὸν πρὸ τοῦ θανάτου ἧττον τοῖς παροῦσιν ἀηδὴς ἔσομαι ὀδυρόμενος.
thank you.