RE: Rusty stain around screws.
The rust on the screw was at the outer edge of the hole. The screw was perfectly clean where it went through the strap. I used a 10X eyepiece to have a good look at it. I concluded at the time that the rust at that point did not come from the anti twist plate. The rust on the white deck pad was greatest near the hole and became less with increasing distance from it.
I read a text on marine metal corrosion about 30 years ago. I have forgotten most of the details. It went into great detail on the causes of corrosion in stainless steel. It is most inert when exposed to oxygen.
I have given my observations. Reading the post about the Aero it appears that the problem continues.
I fitted Da Kine straps and cleaned the foam with vinegar. Both worked well.
I am now back to where I started.