lxmike
M2 fan.
Firstly, it was best for telegrams as 5 unit codes counted as one word. And letters were simpler to transmit by hand in Morse.
Secondly, numbers are limited to a few thousand combinations (10x10x10x10x10) and letters mean 26x26x26x26x26.
Lastly, these codes were commonplace in the days of telegrams; f'instance Bentley's and C&W's.
I used C&W's a lot for international work when I was a civil servant. Copies of the code books were rare and people who had them would auction them off on retirement. Luckily you could remember the common ones; for example TUNHO meant do you agree? or I/we agree. I could type that in my sleep in the 60's and often did...
Regards, David
thanks David a very interesting insight