The item here with an embossed eagle design, does not appear to be part of a powder flask. Over the years, having dug dozens of powder flask parts, along with a number of complete examples, has provided an opportunity to become rather familiar with various details.
The Powder Flask Book by Ray Riling, is included within my reference library as well.
There are several details which immediately catch my eye on this piece, to preclude the unlikely association with being a portion of a stamped copper or brass powder flask. First, the material here appears much thinner in gauge to what is normally seen in rolled metal that was die stamped in manufacturing gun powder flasks. Second, the turned edge at the bottom appears to be on a straight plane, rather than a radius as would be noted at the base of a flask. Third, we may note that the eagle detail is very close to the bottom edge, whereas in most cases the design of any eagle on a flask would generally be higher in a more central position. Next, there appears to be a turned edge at the top, following the same horizontal plane as seen on the bottom of the piece. Additionally, the bottom edge shows a compound double turn, not seen in any flask. Finally, we may note the one smaller piece shown on the left side of the second photo, exhibits a semicircular tab at the end, with a small pin hole.
The points brought forward, clearly define to my eye, that the thin rolled metal embossed eagle piece posted herein, along with the unadorned smaller section, are parts of a side plate from a harmonica.
CC Hunter