Such adjectives as belong only to the consonant declension have two endings. Most such adjectives have stems in ες (nominative -ης and -ες) and ον (nominative -ων and -ον). Under ον stems fall comparative adjectives, as βελτί_ων, βέλτι_ον
a. There are some compounds with other stems: M. F. ἀπάτωρ, N. ἄπατορ
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].