Tom Anderson wrote:

Here's another thought: if you had a letterbox on both sides, could
you have two addresses? Do addresses in fact belong to letterboxes,
and not houses?

I take it you know about Baarle-Nassau?

I've just noticed that the Westbury Hotel gives its address as
The Westbury Hotel
Bond Street,
Mayfair,
London, W1S 2YF

I was aware that estate agents have grown Hampstead and Cla'am to cover all
London between them, but I was surprised to find that merely sounding like
it might be a prestigious address is enough to summon "Bond Street" into
existence.