They used to put pennant numbers back in the Song days. You can still attempt to count the submarines by the pennants. Then at some point, pennant numbers are no longer painted. That makes it impossible to determine one sub from the other. In the Yuan days, you resort to minor details like the shape of the sail.
Nowadays, the square shaped sails of the earlier Yuan class have all but disappeared. My theory is that the older Yuans with the square sails have been getting retrofitted with the new curved sails one by one. It would be impossible to distinguish the retrofitted boats from the new build.
