Paul,
In one of book l came across your wine glass is same shape of a 1910-1920 amethyst wine glass. Pontil mark do not tell you the age because most reproduction glass also have them.
Here is how to tell is your wine glass early 19th c or 1920 reproduction;
Closer examination of the underside of the feet reveals that later copy has the pontil mark indented in relation to the foot( Concave), which also has no visble marks of the tools. If you hold the glass upside down and look at the foot tilted against the light. You can see how perfect it is.
On the genuine glass the foot is much flatter, the pontil mark is more pronounced and there are irregulararities left by the glassmarker's tools.
Do you like to see what old wine glass ? I can take some photo of my early 19th c glass to show you what l meant.
Good luck. Let me know the result of your finding?
Ming