It all brings back memories of when I was taught by a HAM enthusiast friend the delights of listening (and transmitting)back in the mid-late 1970s: on ex-military equipment. I remember the Lincolnshire Poacher, though I haven't heard it for many years now. I haven't got the space at home for the equipment and when I used to, I got too much interference when using my 30 metre long enamelled wire aerial. Those were the days! Transmitting morse at... er... 3-5 words a minute.
They were very interesting to monitor as a teenager anyway. I always assumed they were Enigma transmissions, which of course are transmitted in 5-number sequences. In 1985 I bought my first proper computer and has some shareware Enigma decryption software - but still never managed to decypher anything!