Popular Shell design ,with a fern pattern below collar this style was used in Europe as well as the States but The American Flask &Cap Co kept it on all its Shell design  flasks it was called The Pendant Fern.The flask is a large one about 9.5 ins. It is in good condition for its age ,the secondary rings I do not think are original.but I feel the ones actually attached to the flask are. There are no splits or dings just a small one on the base as you can see in the photo. This is a good mid 19th century flask considering it came through the Civil War.   See photos. Still has quite a lot of original finish.