Thanks so much for the info!
I'm still a bit confused though.
The air pressure thing I can get my head around. BOAC used to have square windows, but 90 degree angles in a highly stressed pressure chamber wasn't a good idea and it cost lives (amazing they could figure out how to get all those tons of metal of the ground and not realise that - but then most things are obvious retrospectively), so anything that will relieve pressure on a flat shatterable surface is obviously a good idea.

I think the main reason that I used to think that the hole was a metal screw was because little frost patterns usually congregate around it - so I guessed the metal screw acted like a metal spoon does when you pour hot tea into a glass, and was there to conduct temperature. Now I know it's a hole, that changes things. If there was a hole that small in a car, it wouldn't make a jot of difference to the condensation levels. So I guess some other forces are at work:

Does the difference in pressure help draw the water vapour out of such a small hole?
How does water get into the sealed gap in the first place? And what stops it accumulating more and more with every flight?

Sorry to be such an idiot, but this sort of thing becomes momentous on a 12 hour flight.

There are over a million parts in a modern aircraft, and I have to go and obsess about a non-part, or rather, the absence of one.