Many thanks for that information. It's as I half-guessed, it's not the lowest point therefore doesn't actually offer a solution.

So it looks like syphoning or a suction device.


I think the reason later plugs seize is partly the heads are smaller, so you can't apply as much force, but also they have a flange like a kind of one-piece washer, and this too creates friction and a place where corrosion can bind.
I've had problems elsewhere removing bolts that have this feature. Older designs using traditional separate washers were much better.