We don't have synonyms for altar piece yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are altarpiece, altar-piece and after piece.
"I walked across the concrete to examine the altarpiece of the atomic age. A large crack ran through it. — Atomic John: A truck driver uncovers secrets about the first nuclear bombs, David Samuels December, The New Yorker, 15, 2008"
"I walked across the concrete to examine the altarpiece of the atomic age. A large crack ran through it. — Atomic John: A truck driver uncovers secrets about the first nuclear bombs, David Samuels December, The New Yorker, 15, 2008"