There is a sticker on the magnetic stripe. Sometimes changing the bar code in the supermarket will be iron. You can tear it off and see a piece of metal behind it. There is also a metal strip. If you go to Xinhua Bookstore to buy books, there is usually a metal strip with double-sided tape on it. The principle of access control is the same as that of hand-held metal detector, but the scanning range of people is larger. You have to see where it is posted. If necessary, the more secret, the better, because if you take this off, it won't be magnetic. I hope Xinhua Bookstore can help you. Some metal strips in your book are sandwiched in plastic seals.