It is not often that one digs up a historical flask - this is a Baltimore area glassworks product from the mid 19th century. It is known in the McKearin flask list as a GXIII-8 (Group 13, Number 8)
Condition is absolutely everything to value. Hard to see cracks or fissures, bruises, flakes all whittle away. Staining in the glass also detracts. For this mold, the clarity of the impression is also important - it seems that the banjo player side is a bit weak. A value of a few hundred dollars is a rough guideline