You do encounter that on occasion. I've seen very fine wire used as well. I suspect it was done because the sac kept sliding off the sac nipple when they put it on with shellac. It happens and is frustrating, but the string really isn't necessary.
When it does happen, take the time to clean off all of the old sac. I usually take some 180 or 220 grit sand paper and wrap it around the sac nipple and give it a few twists. - enough to rotate the section a couple of times. This evens out the surface of the sac nipple and scores it just a bit so that the sac gets some traction and stays in place. This is why many sections originally had two or three rings cut into them. If there is shellac on the inside end of the sac, clean it off with alcohol sing a cotton swab, and dry. Then wet the sac nipple again with shellac and slide the sac on. You may have to hold it there for a minute or so, but once it sticks, you'll be OK. Just give it a while to let the shellac stiffen before you assemble the pen.
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