In my book
THE POWDER FLASK BOOK by Ray Riling, Copyright MCMLIII, published by Bonanza Books, the author shows a horn and several metal flasks with a valve that is very similar to yours on page 443 (ref #1535 flask, 1536 flask, 1540 flask and #1543 horn).
This is in the chapter titled MODERN.
The the horn, flasks and valves shown were made by Mark B. Burnham of Georgia.
The author says the valve was a "...distinct innovations for dispensing powder. Mark states that his metal flask bodies, as well as the valve chargers, are completely made by hand without the use of any power tools...".
As the books copyright date is 1953, and at least four powder dispensers with this type of valve existed prior to that publishing date,there is a fair possibility that your valve was made prior to that date.
I expected to find a patent on this valve and as the book shows a large number of British and American patent drawings I hoped it would show it. It didn't.
Although it didn't show a patent drawing, at least you know the name of the person who probably made it.
Zonie