I have quite a few brushes with black ferrules that are celluloid. Most are hard rubber. But the ferrule on that brush is clearly faux ivory or ivoroid, a celluloid copy of ivory. It is quite stunning with those curved layers.
I thought all the brushes with the clear plastic bases were lucite until I got this brush:
The base is amber clear celluloid and the ferrule is cream celluloid. Same materials as yours with a different twist. This is one of my favorite brushes, I think the combination is lovely, and I'd say yours is even more pleasing.
This combination is not common in my experience. Good thing you didn't overheat either part as you were cleaning it. I had a celluloid ferrule turn to mush in my hands when I hit the inside too hard with a sanding drum, lucky it didn't light up.
By the way, the base on yours looked green to me, but it must be said that I am partly red-green color blind so on closer inspection I realize it may not be green, but it is stunning nonetheless.
It didn't take much to polish, honestly. When I bought the brush from the antique store, everything looked kind of funky. When I got it home, I got a wet rag with a little Dawn dishwashing soap on it and that was enough to clean it up. Just to make sure I wasn't missing anything, I took some 3M rubbing compound on a dry rag and polished everything up. I'm not sure how much of a difference it made, but I thought it couldn't hurt.
I didn't have to do anything labor intensive to it, it was actually a really easy restoration. Sorry I don't have anything more dramatic!