Tumblers often seem to have a large ground pontil and I wonder if that is due to the way they’re are blown. Is the bottom of the unfinished tumbler more likely to be slightly convex requiring grinding of a larger area whereas is it easier to produce an applied foot slightly concave, only requiring a smaller area to be ground? I can’t remember seeing a stemmed glass with a ground flat foot before, is the one that’s ground flat the same diameter foot as the others?
In Bickerton’s Eighteenth Century English Drinking Glasses it says that ‘by the beginning of the last quarter of the [18th] century the faceted stem had almost completely captured the market : it reigned until the early years of the nineteenth century...’ so there were plenty about. Later, in the early twentieth century, according to Miller’s Collecting Glass, facet stem glasses were reproduced by Stevens and Williams and Whitefriars (probably among others). The reproductions it says are heavier weight, have thicker stems, flatter and thinner feet, and brighter colour. It says ‘the laborious technique of hexagonal facet cutting was also rarely used on such pieces.’ Your glass has the hexagonal facets, the ones in your link diamond facets, then there’s simple flat cut facets. Don’t suppose that rules anything in or out though

I am always slightly wary when there is a near perfect set of glasses claiming to be almost 200 years old, especially when there are known to be later reproductions, who knows!
Antique glasses make the booze taste better! (Unless you break one).