Hi Trucker,
I bought some from a german seller but can't find thrm at the moment. But when I looked at photos of the SIR WILLIAM HILLARY the used one seems not too large. 38mm would be 76cm in original, something to put on the back of a boat but not in the small mast. 25mm would be 50cm which looks reasonable, maybe even 20mm but I would choose 25 from that range offered.
My options where 20, 28 and 38mm at the rope-side (upright) - 20 should be not more than 30mm long, that must be the one I've chosen + the 38 in height, I'll use that at the boathouse later.
Best,Joerg
P.S.: Still far from hanging the flags .... (maybe on the boathouse

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