In my HVACR efforts, I often use a tool commonly called an anemometer.
The anemometer is an instrument used to measure wind speed. I have a small pocket-sized version which also has other weather station features that I've used while doing load development and shooting exercises.
I own a couple older versions of said tooling which are marked ventometer.

Evidently, this Jeffrey tool is a wind(-age..?) measurement device.
Expecting to see more pieces of the tool, I did a book search for proper vintage text.

I found this image of a similar device in Sharpshooting for sport and war
By William Wellington Greener...

Also, in A dictionary of military terms
By Edward Samuel Farrow
I found this definition...
"Ventometer.—An instrument used for marking on the sliding bar the allowance to be made for wind. It is worked by a screw in 1/150ths of an inch.

I'll keep digging, but from this much it seems that there might not be anything more to the tool than what we see there.
I'd very much like to see the directions for use!
Perhaps a patent search is in order!

Cheers
Tinker