The string is for adding a bookmark to the leather. You can add it into the tin clasp and tie a knot. I know this was recorded months ago. Are you still using your black TN? I enjoy the modern inserts. They seem to be decent quality.
I put my cardboard craft folder at the back of my TN, and put the pen clip on one side of that. It stops the leather cover being marked, and keeps the pen within the edges of the TN cover. I have some sticky notes and a ruler in that craft pocket and the pen clip keeps it closed stopping the ruler etc dropping out.