Welcome to the forum - you an authentic earlier 19th century flask - this is McKearin mold GIV-28 (group 4, #28)
There appears to be a faint pontil mark on the base - sometimes they are like this.
This one has a good strong color - often they are paler - which adds to the value. The "high point wear" seems fairly significant which hurts the value. Any small flakes in the glass, even if very small, also affect what someone would pay.
Value-wise I am thinking in the 500-700 range.