A quick look thru in Ray Riling's book THE POWDER FLASK BOOK shows a number of valves that use a lever which operates a sliding gate which passes across the outlet.
These were used for both powder and shot flasks.
Some of them only have a gate near the outlet while some of them have two gates with one gate close to the flask and one gate near the outlet.
Normally, this style holds the outlet gate closed and while it is in this position, the rear gate closest to the flask is open.
When the lever is depressed, this style "cuts off" the feed from the flask by moving the rear gate across the supply from the flask while the front gate is opening the outlet.
The earliest date I see in the book for these is around 1850 but of course, the book can't cover all of the Patents so it may have been an earlier design.