Dear Forum members,

From time to time I find here on the forum mentions about the "rare mileage scale".
But I have not found more information about it, i.e. whether it was printed on all watches meant for the US market (I suppose, based on many examples, that it actually wasn´t, and most of them had the classic "base 1000" tachymeter scale), or whether it was on watches for the British (or other "non-SI units") market and why is it considered "rare"? How much rare?
I am asking out of pure curiosity, although yes - I do own a nice UG Aero-Compax ref. 22546 with a mileage scale (but definitly not planning to sell it).

Thanks a lot!