Kovnik as a military rank sounds like GW failing their linguistics check (at least if they wanted something related to Polkovnik), because Polkovnik's root is "polk" (regiment), while "kovnik" sounds like it should mean blacksmith (root "kov" from the verb "kovat"=to forge + suffix "nik" (compare to "ohotnik"=hunter, "volshebnik"=wizard, "lesnik"=forester)). Slavic rank name comparable to a captain would be Russian "sotnik" (commander of a hundred) or Polish "setnik" (same meaning, but I'm not sure how widely it was used). For example, sotniks became captains in Russia in 17-18th centuries.