looking again at this glass, plus other diamond engraved tumblers/glasses from the 1780 - 1830 period, I think I stand to be corrected regarding my comments on the use of the upper case J for the numeral 1.
This correction may support Christine's comments to some extent.
The majority of glasses which carry some form of wording, plus a date, are wheel engraved and show upper case lettering. Almost without exception the figure 1 is shown either serifed, as in the modern form, or as an upper case I.
But when it comes to diamond scratched/engraved representation of the same digit, it does appear that its form changes and it is shown frequently as an upper case J - the reason being, possibly, that the engraver, who may not have been professional, is simply repeating a style that they would have used in their handwriting - and the style was what we might now call copperplate.
At school I can remember writing an upper case T and F that looked archaic with rather florid intros and curly endings - but we did write proper in those days - real joined up stuff, most of the caps. had up-swept beginnings and often curly endings.
For those interested in tumblers, then John Brooks' small booklet ('Glass Tumblers 1700 - 1900') shows examples with hand engraved script showing the upper case J being used in the date, even as late as the mid 1830's, although by then it was starting to slim down.
My other source of images of tumblers is a catalogue from Delomosne & Son Ltd. (March 2008) 'Rare English Tumblers 1750 - 1830' which was produced as part of The British Antiques' Dealers Association 90th Anniversary Exhibitions. Very worth while buying if you are interested in tumblers, although as far as I can see none of the 87 examples shown is decorated in diamond point - they appear to all be wheel engraved, which seems unusual.
I've attached a pic of another marriage tumbler of alleged early C19 provenance, but wheel engraved, showing the more usual 1 produced on the wheel ...... but perhaps without the rustic appeal of the op's example.
It says William & Mary Birchall 1804, and the engraving is promoted from the ordinary by the quite uncommon small polished dimples midway along the strokes.