Funny, I was just going to start posting.
>>4211931 (lb)
https://en.wikipedia.org/wiki/International_maritime_signal_flags
https://en.wikipedia.org/wiki/Pratique
The current "Q" flag is related to the "Quarantine Flag", but now only the "Q" is flown to acknowledge the ship is safe for inspection.
https://en.wikipedia.org/wiki/Quarantine