12-bolt differential cover, black color and texture?

[71CHEVYSHORTBED402]

This is a picture of my OE differential. I hit it twice with naval jelly. There's still some rust. I haven't used any sandpaper or a wire-brush yet, because I'm not certain of the correct surface. Unless ill-advised, I'll probably soak it in white vinegar overnight.

I hit it twice with aircraft paint remover, then some Jayco garbage. Whatever it is, the black doesn't budge. Could this be it's because a rust converter was used?

Also, the face of it is textured, and the one on my truck is the same. Is the texture original?

Funny, there appears to a dent or two in the center on this pic. I need to take a peak. Edit: Yep. I managed the edges, but I'll hire this out. Not that it's obvious. First time I noticed.