Until 2016, it was possible to add 24 new pages to a US passport:
The U.S. Department of State will no longer add visa pages into U.S. passports beginning January 1, 2016. Previously, U.S. passport holders had the option to pay for the insertion of additional 24-page visa inserts when valid passports lacked adequate space for entry or exit visa stamps.
How were the pages added to the passport? Were they physically sewn to the existing passport in the middle or attached at the end? Or was it done in some other way?