1. Hom. has only -οιιν in the gen. and dat. dual.
2. In the dat. pl. Hom. has -σι (βέλες-σι, δέπας-σι), and in a few cases -εσι, reduced from -εσσι (ἀνάκτ-εσι); -σσι occurs after vowels (γένυ-σσι; for γένυ_σι?). -εσσι was added both to stems not ending in ς (πόδ-εσσι, βό-εσσι, ἄνδρ-εσσι, ὀΐ-εσσι, 274 D.), and even to stems in ς (έπέ-εσσι). Hom. has also ποσσί, ποσί; Pind. χαρίτεσσι, θέμισσι. Tragedy has this -εσσι (κορύθ-εσσι), and so Aeolic, and the Doric of Corinth.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].