A five-digit Spanish station on 3927 kHz has been located approximately 15 miles SW of Havana near Guineo, Cuba. A five-digit CW station on 3690 kHz has also been traced to Guineo, Cuba. Another government source believes Guineo to be a major transmitter site used by DGI (Cuban Intelligence).
Finally, there has been wide speculation that these transmissions are messages which are meant to be decoded using a 'one-time- pad' -- If that is the case, it's very curious to note that the same five digit groups are oftentimes seen to repeat over and again in the same crypt -- and that the very same transmissions are seen to repeat from week to week, and from month to month - so often, in fact, that tape breaks are sometimes noted! When spliced back together, the 'sloppy' handling sometimes results in truncated 5-digit groups - the end result being a mixture of 4 and 5-digit groups in the very same transmission!
These facts would tend to point one away from the one-time-pad concept and support a couple of other theories - suggesting that the numbers are NOT a cipher, but rather a code unto themselves, and that much of this traffic is 'dummy' in nature - broadcast simply to keep a frequency open over a long period of time.
In addition, most five-digit Spanish numbers transmissions are very badly over-modulated, resulting in numerous spurs up and down frequency. When broadcast under such conditions, the numbers (6) seis and (7) siete are almost indistinguishable, making it impossible to copy a crypt without numerous errors.