I have not seen one as of yet. The current concentric shaft has one circular shaft inside the other, with splines on the inner shaft.
As I understand the double D, it is basically a round shaft with flats on opposite sides to give it sort of an oval shape. The top U joint of the current ISS is that type. This is the best picture of what I have that would show that.
Thanks for the pics. I am most familiar with my current one. I often wonder if the slight scrunching sound is coming from the universal OR the steering wheel bearing. Has anyone replaced the shaft and the noise/feel NOT gone away?[/u]