The problem with ported racket is the string must stay near the bottom of the port when you string the racket from the top down. Because the turn table pivot point is below the port in the top half of the racket the string tends to stay in the top of the port. So Prince uses a boomerang to hold the string in the bottom of the port. The S hook holds the string down by hooking into the next lower port with the bottom hook and the string is ran through the top hook. This holds the string an equal distance from the lower port. Problem is that distance is not exactly the same every time so you string are going to be crooked.
The easiest way (for all stringers) to get the strings straight is to use the 2 piece 50/50. That way when you string from the center up the string stays in the top portion of the port because the pivot point of the turn table is below the port. The string will be straight. And when you string from the center because the pivot point is above the ports the string stays in the bottom of the port.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.