I cannot speak for the Norton lapping stone but I can speak for the Naniwa lapping stones. Kinda the same in appearance.
With the Naniwa lapping stone there are burs on the edges you should be aware of that need to be filed off. Those little burs, pebbles, tend to fall off as they become wet. They are very small grains and when introduced to the hone during lapping all you feel is a slight tug and nice little scratching sound. Once I filed off the burs, it works fine but does not lap nearly as nice as the DMT325 or Shapton DGLP.
This is just a personal experience and does not reflect the overall quality of any Naniwa lapping stone. What takes me 10 minutes on a DMT325 takes 2x or more using the Naniwa lapping stone. Even at that, I still feel and this is just personal, the Naniwa leaves me with a not so flat surface on the hone.
Here are some pics of my Naniwa lapping stone and my Naniwa 220 after a bur dislodged during the first lapping.
Please note, if you look close in the middle in the distance you can see a bur. This lapping plate pic was taken after I filed down one side of the lapping plate.
Again, this is just a personal experience and as a personal recommendation, please use a DMT325.