Jonas is right and in fact you see quite a few of those apertures in older cameras like Olympus XA but also in television (watch out for the bokeh .-).
The blades have a rectangular cutout in each of them and so form the rectangular diamond shape that you see in the bokeh.
Yes, I noticed the overhead flourescent rectangular lights with their prism grid provide a different reflection (i.e. rectangular) than we usually watch for with the specular highlights for bokeh. I still found it useful for studying the 'objects' bokeh rendering at the different focal lengths (mfd to near inifinity).
thanks for the comment!
