As near as I am aware, the numbers that indicate brush sizes offer no real indication of the actual brush size in terms of measurement.

It seems to me that except for realizing that a Number 10 is a larger brush than a Number 4, the numbers regarding brush sizes are rather a poor indication of the measurement of any brush, as they seem to bounce all over the place from brand to brand.

And, I don’t think what country offers the brush has much to do with it.

Dick Blick catalog does a good job of translating the various manufacturers nonstandard sizes in mm or inches.

Good advice from Alerio

Blick shows the width and the length of each brush they sell from the various makers on the listing of all brushes of a specific make and type. Comparing W&N and 2 different #6 rounds from Da Vinci for example:

W&N Long Handle Round, Size 6
Long Handle
6.4 mm Wide
29 mm Long

Da Vinci Maestro Round, Size 6
Long Handle
6.9 mm Wide
25 mm Long

Da Vinci Maestro Long Round, Size 6
Long Handle
4.7 mm Wide
24 mm Long

https://www.dickblick.com/categories/brushes/natural/bristle/

https://www.dickblick.com/products/winsor-and-newton-artists-oil-brushes/

https://www.dickblick.com/products/da-vinci-maestro-2-hog-bristle-brushes/

Just check all the bristle rounds they sell and see what most closely matches what you already have.