If you ever wondered about the Seascape 18's masthead cap, how it and the other hardware is fastened then here's the gist:
The Mast: Outer Diameter is 80mm, Inner Diameter is 70mm (i.e. wall thickness is 5mm).
The Cap: It's a solid hunk of plastic. 20mm think. It's held in place by something like 3M 4200 and the Dead End Eye Strap's screw. There is a little cutout for the upper screw of the halyard block.
The Hardware: The Dead End Eye Strap uses a self-tapering screw, the halyard block uses M5x30mm machine screws with big washers on the inside and nuts with Nylon locks. The Eye Strap appears to also use M5 machine screws and I'd say it's 16mm long or so. On the inside it's again backed by washers and nuts with Nylon lock.