Policy Explanation
Frankly, this policy and its implementation sounded like the process used by folks who wanted their clients to know that they had received an NSL without saying anything about an NSL. Because an NSL is such that a recipient may not even acknowledge that they have received one.
Check out this link for more about National Security Letters
For example, here is an interview excerpt from Nicholas Merrill, talking about his own receipt, and then the lifting of the gag order after he challenged in court the scope of the Letter itself:
NICHOLAS MERRILL: The way I try to explain it to people is that if before I was partially released from the gag, there were 100 things I couldn't talk about, right now there’s probably 97 things that I can't talk about. The main things that I won the right to say are that I was the plaintiff in this case and that I received a National Security Letter.
[source: https://calyxinstitute.org/about/press/2011-national-security-letters-and-gag-orders]
In addition to gagging the initial recipient, anyone they tell about receiving the letter is also subject to all of the terms of the NSL:
Can I talk to a lawyer if I receive an NSL?
Yes, you can talk to an attorney for legal advice if you receive an NSL, but the lawyer is then bound by the gag order just as you are.
[Source: https://www.eff.org/issues/national-security-letters/faq]
So, how to tell someone a message without saying anything?