Palaeofuture has an interesting article on sending letters, the CIA way.
When you file a Freedom of Information Act (FOIA) request with a federal agency, they’ll often send you physical letters in the mail. When I got my first response letter from the CIA, I was a little surprised to see some old-fashioned, anti-spy tech on the back of the envelope. As you can see from the photos above and below, there’s no way to open the envelope without making it clear you’ve been messing with it.
The author has been inquiring about the how and why, and despite the inquiry not being a formal FOIA request got an answer.
The “gummed kraft sealing tape” the agency uses is three inches wide, and the indications from the response to my FOIA request suggest that the agency buys it in 450-foot rolls.
The article does have a part and an order number for the tape, in case you have need for it.