Hi Neil,
I think the Monogram is for a Friedrich Wilhelm, but the monograms of Friedrich Wilhelm II (1786-1797) and Friedrich Wilhelm III (1797-1840) look quite similar to the one of Friedrich Wilhelm I. (search on google for "Friedrich Wilhelm II or III Monogramm", you will find some coins of these kings with their monogram), and usually show no number. So could your glass date a bit later, maybe in the period of the reign of Friedrich Wilhelm II ? My porcelain plates were made in the 1830s, they can be dated by the porcelain mark, and they have nearly the same monogram as your glass...
Mat