Hi Ian,
I spent considerable time monitoring some of these stations, including the five
digit stations in Spanish typically by a female voice, believed to have originated
from Havana. I was aware of their significance at the time and understood they
would be using one-time pads to communicate intelligence.
Some of the four digit Spanish transmissions were reputedly traced to the U.S.
National Communications System (NCS) training facility in Virginia known as
the Warrenton Training Center. See
http://www.fas.org/irp/facility/warrenton_b.htm
I also regularly listened to the Lincolnshire Poacher and Cherry Ripe.
The numbers stations could sometimes prattle on for extended periods of time with
very lengthy messages.
Also interesting to listen to and somewhat briefer were the Skyking and Looking
Glass coded broadcasts to the US Strategic Air Command airborne B52's,
particularly on 6738kHz SSB. You can read a little about the significance
of Looking Glass here in this 1965 article from Time Magazine-
http://www.time.com/time/magazine/ar...840526,00.html