Well, I am sure not an expert on flasks, but here is my take. For those flasks, I would say on the Continent and in Africa, they would have been used through the end of the muzzleloading era. So, up to 1860 - 1870 or so. Most of the big game hunters either used a flask, or a leather pocket on their vest out of which they would scoop a handful of powder to reload. Forsythe mentions carrying powder in a flask in India. Baker mentions carrying powder in a pocket while in Africa.
Frankly, Kermit, I would not trust my hand and my head to the flameproof valve. What's the old saying, if it sounds too good to be true, it is? Now granted, I have never had a load go off while pouring powder down the barrel, but it sure could.