A numbers station is a special type of unusual radio broadcast, generally on the Short Wave (SW) radio bands, reading out incomprehensible lists of spoken numbers or morse coded messages.The most common type features a female voice, reading long strings of numbers, generally in groups of five, often preceeded by a preamble and/or a series of musical notes. During the Cold War number stations were often operated from Eastern Block countries, such as Czechslovakia, East-Germany (DDR) and Russia (USSR) with their broadcasts aimed at spies and secret agents operating undercover in West-European countries. The agents were instructed to listen to specific SW frequencies at fixed days and times, often using a commercially available SW-receiver from brands like Sony, Panasonic and Grundig. The Sony ICF-2001D (called ICF 2010 outside Europe), a commonly available receiver, became really popular amongst the spies during the 1980s.
The Sony ICF-2001D, as a spy tool, complete text http://cryptomuseum.com/spy/icf2001d/index.htm
The first microprocessor controlled travel shortwave radio ICF-2001 introduced by Sony in the year 1980 initiated a revolution in shortwave travel radio design. A high performance set had not anymore the size of a 6 – 10 kg suitcase with a carrying handle, but it came in a pocket book siced cabinet and looked like a giant pocket calculator. The ICF-2001 was the first shortwave travel radio featuring direct frequency keypad input together with several electronic memories and a liquid crystal frequency display. The ICF-2001D (called ICF-2010 outside Europe) was the successor of this successful set and was improved in several ways to make nearly the perfect shortwave travel portable for many years. In the similar size cabinet, it was a completely redesigned shortwave receiver with very high sensitivity, good selectivity, a synchroneous detector for automated ECSS reception, a large number of memories, ssb and cw reception, scanning functions, a clock with timer – an in some non European variants, it also covers the VHF Air Band. http://www.shortwaveradio.ch/radio-e/sony-icf2001d-e.htm
Sony ICF 2001-D, pdf Manual http://www.shortwaveradio.ch/radio-manuals/sony-icf-2001d-manual.pdf