If this is the part you're looking for (see below), it's the threaded plate behind that panel and was part of the "Body Lock Pillar Reinforcement Panel" installed by Fisher Body, I believe the service part number was 4469685 ("Plate, Door Striker Anchor"), but I'm not positive about that, and it's been long since discontinued. I also don't believe anyone is reproducing these, so your only option is to find one used.
Ed