A lot has changed since then but they had techniques to identify the listeners as early as the 50s https://en.m.wikipedia.org/wiki/Operation_RAFTER https://en.

A lot has changed since then but they had techniques to identify the listeners as early as the 50s https://en.m.wikipedia.org/wiki/Operation_RAFTER https://en.m.wikipedia.org/wiki/Operation_RAFTER Having said that there isn’t really anything special about this particular technique of using numbers stations. It’s just a part of the same trick to pass along information via an open channel without having to give away what the message is about or who the intended audience is supposed to be. Taking out an ad in the classifieds section of a newspaper is ultimately the same trick just with a much lower bandwidth to transmit anything useful beyond a simple signal.