Some substantives ending in -εως are placed under the Second Declension because they are derived from earlier ο stems preceded by a long vowel (-εως from -ηος, cross34). A few others have a consonant before -ως. The vocative has no special form.
N.—This declension is called “Attic” because the words in question generally show -ως in Attic and -ος in the Koinè dialect (p. 3, F).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].