A Civil War era flask would have an applied top. That one appears to be tooled, which would make more like 1890 or later. Those strap sided flasks were very popular in the late 1800s and early 1900s. Still a nice find. The '19' is probably the mold number.
Thanks! I attached a close up of the tooled top, it was buried in soft dirt beneath a floor which was above surrounding grade so remained dry. It was not uncommon that an S would look like a 9 or vice versa?