I can only help with a few things, so maybe someone will come along and help with the 543 port number.
Unless I'm mistaken, the "S" stands for Seaman, and the "12" means that it was filed in the December schedule. There are no numbers as high as 523 that I can find referenced anywhere. What you're Grandfather's name and age, and perhaps someone else could take a look at it. As for Port Rotation Numbers, the NA claims that no legend/key has ever been found for them, so there is no current way to identify the ship by using that number alone.
Hope this helps. I'm just learning these things myself.
Janis