Morgan, there are precious few modern sailing boats that can sail inside 40 degrees off the wind so there is no point whatsoever annotating that area on a wind gauge as you simply can't go there. In any case wind indicators are so imprecise such accurate markings would be a bit superfluous. No military sailing vessel would ever have had such a gauge, there is almost a century between such technologies. Such a connection is, as I tried to allude, overly fanciful. How many more reasons do we need for this to be nothing to do with sailing?
I can see no evidence of such extraordinarily selective corrosion that might explain 2/3 of one side being surgically removed - corrosion just doesn't work like that and as it is presumably behind a glass face it probably hasn't been removed physically. As you say wind indicators operate through 360deg, this one appears to be asymmetric.
Millirads; granted, so it's probably not naval artillery.
Still none the wiser.