I know the S50 also left the factory with a plastic impeller. Having seen it in both it and the M44, I've been under the impression that the plastic pump was common across E36s, at least for a few years.

They were plastic, then metal and from what I understand they are a plastic composite again.

When I pulled out my original pump last month, it had a silver metal impeller. The one I bought from Circle BMW was black, hard to tell if it was plastic or metal. I asked my GF what she thought, and she said it was metal, but really hard to tell.