I didn't say you couldn't sail within 20-25 degrees I said you can't sail much closer than that to the wind
depending on what you're sailing. Perhaps the markings are used to high light the zone.
I'm pretty sure the instrument in question won't have been fitted to a high performance racing yacht either.
There is quite obviously some sort of corrosion on the lower left quadrant not apparent on the other three quadrants.
then why annotate the dead zone so accurately when it doesn't need annotating at all?
Err, because it's a wind indicator and they display through 360 degrees. I think someone mentioned that earlier.
Naval artillery uses milli rads.