This is an excellent example of an early 19th century, English, hand blown, Bristol green wine drinking glass or ROAMER which we date to the George 111rd Regency period, circa 1815.
These glass ROAMERS are fairly rare. The glass has a cup bowl with a thick stem, having three heavily milled prunted knops, on a hollow finely ribbed foot The underside of the foot has a good snapped-off pontil mark, evidencing the hand blowing process
We date this glass to the early years of the 19th century, circa 1805 to1825.
 
Diameter
2.63 in
Height
4.9 in