An artifact* printing plate? Now I think I've seen every thing.
* I'm trying to remember to use the term "artifact" or "relic" for any game-used, event-used, seat cushion, bat, helmet chin strap, show lace, whatever piece of memorabilia they cut up into a tiny 1" square to glue onto a card instead of calling them "Game Used" or "Event Used". It seems to cover everything now, and avoids the "was it REALLY used in an actual game?" debate/argument.