28/29 vs 30/31 phaeton top irons

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These pictures should give you some idea of how the stauntion attaches to the 'top bracket' which in turn screws to the wood bow at the junction where the metal bow meets the wood section.
From memory there is not a lot of difference between the 28/29 and 30/31 top bows. I think the overall expanded length is diffetent or longer on the 30/31.