Thanks Clive, I did wonder why the static ports near the doors used those plates.
I'll look for a photo. If I can't find one, I'll take one next time I'm with G-BOAC (next week).
There are others as well, as in the pictures below (but these are not the ones I'm referring to):
https://www.google.co.uk/search?q=co...%3B3912%3B2599