I wanted to report back on what I tried and what worked and what didn't work on cleaning these up.
These backing plates were not in all that bad of condition to start with. It was like the surface had oxidized and turned a dull flat gray in some areas. It was also rough in texture when you ran your finger over it.
Here's what didn't have any effect on them at all: powdered dishwashing soap....vinegar.....milk and baby powder.
Before buying any polish, I decided to try some Brasso that I already had. That worked! I also tried some fine grade bronze wool. That also worked. So I decided to try a combination of both. Applying the Brasso with the bronze wool. I think it worked about as good as anything could. It didn't make them pristine but it made the outside look like the areas inside that had been sealed up and never exposed.
Doing this did not leave any scratch marks unless you pressed really hard. Where I had tried Scotchbrite earlier in a hidden area, that definitely left scratch marks.
After the Brasso, I washed them and tried the suggested buffing with a plastic scrubbing pad. That added more shine to them. I was surprised by that. I didn't think it would do anything. I finished them with a coat paste wax inside and out.
The before and after pictures don't look much different but the difference is there. The before picture makes them look better than they really were. The after picture doesn't do them justice.
Over all, I am happy with the way they came out.
Thanks for all the suggestions................Dave