Haven't listened to these things since I graduated from university (my last paper was on differentiating pirates and clandestines, and I think I had 'numbers' in the discussion. It'd be interesting to know what if anything of the sort still graces the SW spectrum. I do vaguely recall hearing speculation that some numbers signals were located to Langley VA area. I doubt only one side ... Chrisphen, thank you for that link. Nice!
