It would seem to me that, if these were Christian from that time, they would show more of the central story of Christianity, ie. Jesus on the Cross, Jesus and the apostles, the nativity, etc. If they show Old Testament stories, it would seem to me that they would more likely be Jewish (or lost tribe).
Where are you seeing the 500 pieces of Han carvings? I clicked on the article source and only saw one. Could the other 499 have such symbols?