Threshold of the door?

[SRH916]

Havent seen this truck yet, hopefully will be in good enough shape to pick up, and start a new project. Only thing I get as far as rust with out seeing it is this:
"What I was originally talking about was the piece on/in the cab of the truck "threshold" it seemed as if what you could see was around the silver piece."

I asked about rocker panels, but not exactly as knowledgeable as some of us are, but the only thing I can think of from this description would be a sill at the rocker panel, any other ideas? vin plate? I can't pinpoint this one...

Thanks,
Jared