Unknown Part Help

Under the hood of my 79 B is a rusted body part that I can't seem to find in the moss catalog. On the drivers side of the car, under the mastercylinder, Clutch mastercylinder is the part. Is this the sill, the side member panel or the rocker panel? The moss catalogue shows a cross section of these parts, so I'm not sure which is the part I need (maybe I need them both?). The part has an inward curve (towards the motor) and runs down to a sill where the brake fluid has been collecting for a while and rusting everything.

Firewall sill? maybe.

I think the firewall sill is a little further back. How about this: Ifywhat would it leak your master cylinder leaked, what would it leak onto? What would the brake fluid collect on?