The Lexicon Rhetoricum Cantabrigiense s.v. ascribes this information to Philochorus (FGrH 328 F64); see also Pollux 8.94 and Harpokration and Photius s.v. for the basic didactic/polemical point that the nomophylakes were not to be confused with the thesmothetai. The LRC also claims that they were created in 462/1 BCE, in or as an immediate consequence of the reforms of Ephialtes. For several reasons (notably their association with the proedroi, who did not exist then) this has generally been felt to be improbable. Harpokration cites two lost speeches of Dinarchus which mentioned them, and if these speeches dated from the second half of the 320s it would become more understandable that there is no mention of the nomophylakes in the Aristotelian Athenaion Politeia - whether they are an institution of the last phase of untrammelled democracy (c.327-323) or a creation of one of the alternative constitutional regimes which came later, that of Demetrius of Phaleron (317-307). See however O'Sullivan (below) for an acceptance of Philochorus.
cf. nu 488, nu 489.
[1] Who of course included the (six) thesmothetai. See theta 267.
[2] Bathing this ancient relic in seawater was one of the ceremonies in the Panathenaia festival (pi 151, pi 152).