Stanchion fitting?

[nigelmercier]

I've had a leak from one of my stanchions for some time, so I started to investigate today. The underside has a pair of nuts, but I can only get one off as I need a deep 19mm socket. I'm not sure how this fits together, what will I see when I manage to get it apart?

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[alahol2]

I imagine there's a welded flange sitting on the (fibreglass) deck with a threaded portion passing through the deck onto which your nuts are threaded.