And welcome to the association Alan - I hope we've solved your query!
As an aside, I'd suggest that queries such as this are best posted in the forum, it's more suited to discussions etc. and I think more members follow the forum than other pages. You can easily attach photos to forum posts too.
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Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
It looks like a thing for holding fenders. You attach it to the guard rail. Maybe?
You're right Simon, specifically it's one of these (a picture shows it the right way up):
http://www.marinesupplydock.com/Sea-Dog-3273571-Rail-Mount-Dual-Fender-H...
And welcome to the association Alan - I hope we've solved your query!
As an aside, I'd suggest that queries such as this are best posted in the forum, it's more suited to discussions etc. and I think more members follow the forum than other pages. You can easily attach photos to forum posts too.
Rick Jones (Treasurer), former 26X & 19 owner, Isle of Wight
Ah yes, I can see that now. Thanks for the link and the welcome.
Al