Here we go-

Definitely room for vernacular there.
It's a windage adjustment tool.

From The book of the rifle
By Thomas Francis Fremantle

There are small instruments commonly known as ventometers (fig. 79) (a misleading name, as they do not in any sense measure the wind, but only assist in applying a correction for it), which are used to mark the wind line accurately on the straight bar. These are of various patterns, but all depend upon the same principle. They are placed transversely to the back sight, and bearing upon the outside of the uprights. By moving a screw, a small, sliding square is brought either to the centre of the bar, or to any other point, and enables a line to be marked in precisely the right place, the distance being measured by a little scale, similar to that of the vernier, and giving similar divisions of f^ of an inch. These are very useful little instruments, and are used by a large number of shooting men. They are sometimes fitted to the elevation vernier itself, as in fig. 80.

My guess is that it's used in conjunction with either other tooling (proper anemometer) or by comparison of sighting shots.
Patent searches will likely tell the rest of the story.
Cool tool!

Cheers
Tinker