I think Schreiner's iris named Superstition is not at all related to the iris of Dioscurides. Was he speaking of a type of iris, or of a specific iris? Schreiner's is just one that was hybridized by them and given that name back in 1977.
I am afraid that what his iris actually looked like is lost to history. The earliest bearded iris that are still around today, or at least the earliest with some sort of positive identification, are from much later. Florentina is one of them and is from the 1500's (more or less). There may be many other iris species that have been growing since the time of Dioscurides but the history is unknown for so many of them. A European historical iris society may be of better help, and you can check with HIPS (Historic Iris Preservation Society) as well.
If you do find out more about his iris, please post your findings. I think it would fascinating to know more about the flower we love so much on this forum.
