RE: Rusty stain around screws.
This is poster 1 again.
The stain in this case is Iron Oxide (rust) from the screws. I first thought that the screws were a low quality stainless but a very strong magnet would not attract them. I think that the rusting was the result of crevice corrosion.
This is where part of a stainless object which is shielded from oxygen develops a different voltage from the part which is exposed when immersed in an electrolyte (in this case salt water). Biological activity or and corrosion could have lowered the oxygen in the screw hole.
When I got this rust stain developed, the board had been left in a bag and there was evidence of rusting at the edge of the hole but nowhere else except of course from the anti twist plate which corroded badly. This may have made a further contribution to the galvanic cell which caused the rusting at the air boundary.
I was able to remove the stain quickly using a brush and vinegar.