Antique Silver Sheffield Plate 851 6 Pc Coffee/tea Set Tray
Old Sheffield Plate: An Introduction (Antique Silver Article)
Miller's Silver & Sheffield Plate Marks: Including a Guide to Makers and Styles by John Bly
Sheffield plate, its history, manufacture and art; with makers' names and marks, also a note on foreign Sheffield plate, with illustrations : Veitch, Henry Newton : Free Download, Borrow, and Streaming : Internet Archive
But it is a general rule that virtually all pieces marked 925 or sterling 925 are modern.