The aperture is a massive 21 1/2" wide by a jaw-dropping 4" deep, that's in the old money of course (the alternate, modern, measurement of 55cm x 10cm somehow lacks resonance for me).
To put its size into perspective; a regular domestic letter-plate normally extends for no more than 8 inches and is unlikely to be deeper than 2 inches. Well, using that scale of measurement, this one sounds like a real biggie to me and it can be found here:
Yes here, above the entrance to the lobby of the prestigious West-End offices of Linda Internet (and an associated mini-cab firm) at Number 16, Manette Street, Soho, stands this postal colossus.
But why is it above the door? And why is the thing wider than the door itself?
It's so big one could potentially post a whole stack of LPs, even diagonally, into Linda's void. That little knocker still works, even if the plate itself has now lost its function.
It looks and feels early to mid 19th Century to me but the staff at Linda's were unable to offer any detailed history about this little known London oddity. Can you, dear reader, offer any proper explanation?
Or, perhaps I am wrong and an even larger letter-plate exists within our Capital, if so, please do share its location.