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Good very fine.

Sorry, incorrect answer. Should read it could happen the other way round, all dependent on the metal alloys used. Whatever, the key to having a solidly held central section depends on tight tolerance machining of the two blanks and sufficient temperature divergence from those normally expected to drop the middles into the outer ring, before putting the design on. Nobody is going to align two pieces consistently to make it a plausible mint error. This was demonstrated soon after they came into circulation, with videos to prove so.