That's it!. I have been trying to figure this out for a few years now. I always thought that it was a pinstrippers brush, but I tried it and had problems with line thickness. And it is far easier to place the brush into the inside corner of the lug rather than offset a little like in the pictures from @
xiaoman1 above.
I got so frustrated on my wife's Panasonic that a bought a paint pen and put some ham fisted thick lug lining on it that I wasn't too proud of.
The drafting ink pen is the trick. Thin it so the paint flows. I have one somewhere around the house. Now to find it.