Dwardo, I think that was a precursor to the Siper. I found this on ATARNet forum (Horn Bow forum)
chang al-baz This was an unsupported overdraw where the arrow head slid back along the thumb onto the area of skin between the thumb and fore finger.
It required the utmost skill and steadiness. Without total concentration, a slight vibration in the body could send the arrow's tip into the belly of the bow or into the back of the hand. I personally think that the siper was developed to extend and control this kind of draw. Perhaps there was some analogy to the arrow guide used to designing the first siper, but it was not necessary.
It serves as a channel or groove for an arrow shorter than the bows draw length. I've seen some pretty fancy pics of the more modern Siper.
