It looks like like old mold prep to me. I would try it out and if it drops nice bullets then leave it alone. I picked up a H&G gang mold that looked like this and it works beautifully as-is.
If there's build up on the flat surfaces that impedes full closure, I would scrape it off with a razor blade, then scour it a bit with oil free 0000 steel wool to open the vent lines. The cavities would probably remain untouched.
