We bought from eBay glass fish-shaped plates, manufactured by Arocroc (France) in 1970 or so. The listing had "unused" in the description but that is not certified. How does one kasher glass plates?
The Sephardic opinion is that glass never absorbs, so it never has kosher issues.
Ashkenazic opinions vary; a lenient (albeit very authoritative) one is that of Rabbi Moshe Heinemann of the Star-K:
But as always, ask your rabbi!