Thanks for checking out my customization thread. You're like Astromech royalty IMO. Your site is a fantastic resource and so informative. So glad you are keeping it up.
Regarding the grey panels on those two droids, I have the grey panel on the Black and Blue Droid but not on R3-T2 because I did not see any picture showing this. Do you have a pic or any info that you can share?
Here are more pics of the Black and Blue droid
Can't give you a direct pic, but I think it's pretty obvious that R3-T2 is just a dome swap with R2-D2, and there are plenty of pics showing R2 having the grey square on location. It's an important tracer in telling which motorized body was being used in which scenes.
See every astromech droid @ http://angelfire.com/droid/astromechs