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I can infer that they are intended for aircraft to recognize the Greyhound, and for the Greyhound to confirm its identity. But do these code noun phrases signify anything more? They appear twice in Greyhound (2020) - SCRIPT - Scraps from the loft.

Message from the Admiralty, sir.

Some of it’s badly scrambled, sir.
I’ve done my best with it.

XO to the bridge.
Aye, aye, sir.

What are these scrambles?

I think coded numerals, sir.
But I’m sure of that.

“Expect aircraft.

His challenge, ‘Uncle William.’
Your answer, ‘Baker Dog.'”

Second instance:

[lookout] Second sub spotted!
Periscope, starboard bow!

[lookout] He’s got us lined up, sir!

All mounts, local control!
Get those guns around!

[lookout 2] Air cover! Air cover!

[airplane propellers whirring]

Signal Bridge reports aircraft signals.
“Uncle William,” sir.

Reply, “Baker Dog.”

Right standard rudder!

Mounts 41 and 42,

open fire to mark the target
for that aircraft!

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No, they do not signify anything more than a standard challenge and response password bona fides. Neither the challenge nor the response have to make sense. Often times, when the meeting or interaction is covert, the phrases used are such that they can be worked into normal conversation. In the case of a merchant marine convoy being escorted by military warships, the need to be covert would be pointless. Signal security would take precedence of subterfuge. Having a challenge and response that would not be easily guessed is more important.

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    It may be worth adding that all four words are from the phonetic alphabets in use during World War II. The challenge and response are essentially UW and BD, and may have actually been signaled as individual letters in Morse code, rather than spelled out with words. Those are probably randomly generated letter pairs, for the reasons you describe.
    – Blckknght
    Jul 27, 2020 at 22:50
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Uncle William shows a two-letter signal UW which means "Welcome". Baker Dog shows a two-letter code BD which means "I have established communications with the aircraft". See the International Code of Signals.

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