Genitive of písmeno

I don't know better, but I suspect the genitive of the Czech word písmeno (letter), a neuter noun as (all?) others ending in o, can be písmene alongside písmena, the latter the regular form, due to contamination with other ne genitives, but of masculine nouns, as kmen, kmene (tribe, trunk) and hřeben, hřebene (comb).