I was rather pleased to find out that my motorsailer's 6 x 9 inch "tree stump" mast was in fact a Selden, so I purchased a Selden pole lift ring to fit in the handy grooves. Now, of course, I have to tap in holes for cheek blocks (or cut rectangular holes for sheeve boxes) to move the thing up and down.
I have a further question out of these Selden pages, though, Alex: See here?
Everyone I know tightens the turnbuckle with a screwdriver. Why a wrench/spanner?