The Identi-pen is made by Sakura and is available in art supply shops. It was tested by Mariana Munyer and written up in an “How To” article for the Illinois Association of Museums (“How To…Mark Objects in Museum Collections, Part One: Barrier Coats, Pens, Inks, Paints.” Illinois Association of Museums. October, 1997, #17. Part Two (#18) covers tags, direct marking, and difficult materials.)
Here is what she says about the Identi-pen: “This pen comes with two points (extra fine and fine). It writes well on acrylic resins, and does not smear when top coats are applied. The extra fine tip is better for marking purposes than the fine tip. Identi-pen is available from Gaylord Archival Products and from some art supply stores.”
You can order the “How To…” articles from the Illinois Association of Museums for $2.16. http://www.state.il.us/hpa/iam/publications.html.