What size knot???

I picked up two vintage shave mugs this weekend, and the store owner gave me two old shave brushes, I do not currently have a badger brush and I was able to get the old knot out of one, its nothing special just a no name two tone black and whit handle brush, but I would like to try my hand at restoring it.

With my calipers it measures right at 21mm, and there is a 21 embossed on the inside of the handle after chipping all the old residue away. Some sites say for a proper fan go 2 sizes smaller, so I would need a 19mm knot is this correct? The old knot measured 21mm.

Yes, a 19mm should work. Although many knots above the plug, at the hair tuff, is a few mm bigger. A 18mm might be a better choice for fitting.

One thing you can do is call thegoldennib.com and ask them what they think would work best. They will measure different sized knots for you, right on the phone, and make sure you have a proper fit. Their customer service is terrific.