When I spoke with the archivist at Warner, Mr. Leith Adams, he said this car was used for Batman and stored away, it was not repurposed for Batman returns. Once something is in the archives, very rarely do they let it out to be used for anything again. In recent years Warner has been trading and buying pieces back that they let slip through their fingers.
While the car looks satin, that's just my impression. It could be a semi-gloss paint that was very low gloss. Leith said that they wanted the car to soak up the light and not reflect so it was very stealthy in the film.
It's not flat, it has a slight sheen to it, but Mr. Adams used the words Satin and even went on to say it was a type of primer instead of a paint that was polished somehow to achieve the light sheen.
Now I'm no paint expert and I can guarantee you that Mr. Adams is not either, but that car they have on display is so perfect it make me drool to think about it.
I can say that it's got zero color added to the paint in any way to change colors like flip flop paints. I've seen several replicas that have uneven paint jobs or even look splotchy, but this car is slick as hell and perfect in every way, almost to perfect.
Anyway, I hope this helps and if you live in CA, go see the car when they have them out in the Museum on the Backlot tour. Ask for Mr. Adams and I'm sure he will be happy to answer all your questions.
Dex