You might also be interested in the 2 letter signals for maritime use to be found in The International Code of Signals. The Q signals are mainly for manoeuvring e.g QR 1 - Can I come alongside
The Z signals are for establishing communications as far as ZR - repeat the signal now being made to me by .... and from ZS to ZZ they are the International Sanitary Regulations! e.g. ZV I believe I have been in an infected area during the last 30 days