In the philatelic world, that of stamp collecting, a cover is a card or envelope that has traveled through the mail and contains postal markings. Collectors often prize first day covers, those postmarked on the first day of a stamp's release, or topical covers, those envelopes with a printed, stamped, or embossed design related to a specific topic.
There is even an organization, the Universal Ship Cancellation Society, dedicated to collecting cancellations and covers related to ships and submarines.
The USS HADDO (SSN 604) has many commemorating such events as the launching, commissioning, decommissioning, and recycling. The boat also had, at several times, a rubber stamp that was applied to personal mail leaving through the ship's office.
Examples of both are featured here. This is the current Cold War Boats SSN 604 cover collection. If you have others that you would like to share with your shipmates, please contact your Site Lead or the Lead Admin.