Courtesy of Don Linowski , Fuji 35 'Tenacious' (Hull #60).

In Don's words - "The cover plate is access to the electrical stuff at the base of the mast. It is a 3" hole cut with a bi-metal hole saw, and 1/4" bolts tapped into the mast. Make a rubber gasket out of an old inner tube. I've never seen anyone else do this but it makes good sense. I have tested it for over 10 years with hard sailing. There is no sign it has weakened the mast. It is much cheaper than paying a crane operator to lift the mast every time you need to fool with the mast wiring. And you don't have to re-tune the rig! "