The headword is superlative of the adjective proferh/s (pi 2920), glossed with a comparative in the neuter nominative/accusative singular. Compare Photius pi1375 Theodoridis s.v. prosfere/steron [sic] (with this same gloss), taken to be quoted from Herodotus 5.111.3; also Hesychius pi4069 s.v. profere/steroi (with glosses which include belti/ones), taken to be quoted from Homer, Odyssey 21.134.
Both the comparative and the superlative occur in Homer, Hesiod, and later epic (also post-classical, and especially Christian, prose).