Hi, about a year ago I saw a composite airtwist knopped stem glass for £35 in an antique emporium from a seller who just sold glass. I rejected it at the time as there was zero foot wear, even when viewed with a loop...and I assumed the seller knew their business. I wish I’d bought it now, even if it was a repro as it was quality - I wouldn’t want to make your glass for £20.
In my experience, you need to hold the glass in question in one hand and compare it to a similar glass that is beyond doubt in the other hand. It’s very tempting to believe you have the exception. The trouble is, if someone unscrupulous had bought your glass then they could spend an hour or two rubbing it on a flat stone to add some age. Puts me off spending much on Georgian glasses.