What is this part?

[Dan_P]

I'm getting ready to do a bunch of rust repair.

Does anyone know what part this is? I can't find it on ClassicHeartbeat, GMC Pauls, LMC, Brothers, etc.

I can't find a schematic that matches. It has rust on both passenger and driver side, so my preference is to replace the whole thing.

The picture is from the driver side, rear wheel well, looking forward. Picture is taken from the front of the rear driver tire looking forward. You can see the back of the driver rocker box (with end cap removed).
You can see the rocker box mounts to it (rusted out on left side), and it also attaches to the polyurethane body mount. Runs east to west.