When I was a beginner and I learned the upper body charts
Number 13. was Right Foot Forward with a Right Roundhouse Punch, followed by a Left Roundhouse punch.
I remember us practicing this repeatedly.
The target we visualized was a right then left strike into the temple of the opponent.
The way this movement was practiced was with the standard Isshinryu fist but the roundhouse strikes with the hand turned away from you seeing the back of your hand during the strike, so your thumb was pointed down, your hand turning into the strike during execution.
When the strikes his your first assumption was you were striking with the first two knuckles into the temple area. And the punches were circling up and curving in to make the strike. Almost ending on a descending arc.
We did not practice the strikes against a partner in those days. So the visualization I described was how you imagined the strikes would work.
Then in time when I began to train with other styles studies with attackers began to filter into my training.
I came to realize other things were possible.
The inverted fist strike of our roundhouse strike could also be used as a thumb strike into the neck of someone. Which could be a KO strike because of the area being struck.
Then another possibility presented itself to me.
That inverted striking area could be a double strike using the tip of the index finger knuckle and the thumb. Another possibility
Thus, this could be used for a strike to the temple, or into the neck depending on choice, and with different striking surfaces depending on the effect of the strike desired.
It time (About 1987) I came to realize others in Isshinryu were using the double hook punches as in Wansu kata for this strike. And that was a reasonable use. Just not the one I learned.