I'm in the process of replacing the rear beam on 1747. (PS. for Gary: 1747 rear beam was definitely not replaced!)
Does the Capral section you bought have an external radius too? I've got Nic's boat at my place and his seems to have a small radius on the outer edges also which is a nice detail. ( I think PhilWS beamed his so I would assume the assoc supplied these?)
Is there assoc. stock available and are they anodized? Either way I need to get some beams over the next few weeks.
