The main chain plates are interchangeable. However the lowers are not. The aft ones are built with an angle to mount flush with the bottom side of the deck. The forward ones have a "backing" plate at an angle to fit the space they are in.
BTW, the AFT lowers have a tendency to crack in the middle of the deck where you can't observe them. And I'm told that when they fail it's at a bad time and the mast comes down. Ours didn't but was close to failure.
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File comment: Port Aft Lower
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