Hello,
sorry to butt in once more, but couldn’t it be that, in times past, an apartment used to be called a „flat“ in America, too?
I once read, in Bill Bryson‘s book „Made in America“, a very instructive and enjoyable read on American English and American culture. In this book, the author quotes the following little poem, which was a roadside advertisement from the 1930s for a - then very famous - brushless shaving cream:
„He had the job,
He had the flat,
but his face scratched
And that was that. Burma Shave“
From this, I concluded that, maybe, before the mid 20th century, Americans used the therm „flat“ as well. Maybe…🙂
They are still called flats in some parts of the country. My oldest son has the middle floor of a three-floor brownstone outside of Albany, NY. You can call it an apartment or the second-floor flat.