It tried to find the accompanying brand names and designers. Nothing came up, but I did find what looks like a gem of a book, also titled An Alphabet of Monograms, by Henry Lillie, seemingly published in 1865.
Cover image from the State Library of Massachusetts
It’s a bit grotty, to be expected from a library book of that age, but what a great archive of the decorative nature of Victorian design.
A few more pages from the book on thegraphicdesignproject.