Ca 1775, the likely powder container would indeed be a flask with a lever and spout head, but would most often be made of horn. The flask posted above would generally be identified as from the first half of the 19th century because of the overall shape of the body and spout.
Unfortunately there's not much printed material available with information on these earlier flasks and their use, but Ray Riling's master work, The Powder Flask Book, does have images and at least gives a basic date range for the various designs. In 18th century sporting scenes and portrait paintings, you are sometimes treated to a clear enough view to see that some are of a similar shape to the 19th century pear shaped flask but shorter and stouter. More often than not these scenes show bird shooting rather than deer or boar where you'd expect to see a rifle in use.
These were mostly made from pressed, flat sheets of horn that were formed over dies with the two halves joined and a brass metal band applied to strengthen the joint. The flask heads are similar looking, but the lever springs are attached to the spout, which is a more graceful, turned, tapered tube. Metal flasks existed as well, but the techniques for mass producing horn were much more widely developed whereas the forming methods for sheet metal was not as fully advanced as in the early to mid 19th century. Flasks such as the "Queen Anne" were certainly in use at the time and would have been carried in a coat pocket as very few have been seen with rings for attaching cords or straps. There were other styles of spout that used a removable cap that doubled as a measure.
If you're wanting to have a flask to use I haven't found any available replica or reproduction flasks: in this case you'd need to have a custom piece made. There are originals that pop up for sale and you just need to be ok with using an antique.
I hope this is of interest. Best wishes!
