Thanks everyone for the responses!
I now understand that it doesn't make sense for anyone to manufacture a main plate for this movement that isn't Rolex. I was also able to find a main plate here:
Based on this thread:
That has the same markings. I also understand that the case screw isn't a critical part, and as long as it works is sufficient.
On the topic of corrosion, I have requested a full case picture and pressure test of the case. Stay tuned...