E.g. the oldest grammar, Twe Spraack vande Nederduitsche letterkunst (1584), see dbnl already shows the confusion of masculine and feminine gender in the genitive as it quotes: "des heers, vrouws, diers", while at the same time trying to identify 6 cases as in Latin. The focus on Latin grammar would cause later grammarians to insist that "des vrouws" was wrong and to prescribe that it had to be "der vrouw".
