Thanks so much for all this great help Silvermakersmarks and AG2012!
I saw another posting of the Sileda hallmarks in an on-line auction for silver work on a glass item with hallmarks that closely resembled the marks on the base of the mystery paperweight (other than the date hallmark). I am not posting this image since I didn't take the photo, but you might see the other example of the Sileda hallmarks if you search for this image: "Sileda perfume, scent bottle, Selkirk Glass, hallmarked Britannia silver overlay"
Another thing that I noticed was that Scotcade Ltd. was not yet incorporated in 1985 (although the letter L dating the piece corresponded with 1985). Scotcade was not incorporated until 1987, while Sileda was incorporated earlier in 1974.
Phil hit a bullseye in recognizing the Sheffield and Britannia marks! Many thanks for his sharp eye!