I researched sill plates, and found this.
"In automobiles, the sill plate is located underneath the door and sometimes displays the make or model of the vehicle."
However, I just now looked under the door of my 65 and there is no such animal there. My only guess is that they are talking about the data plate on the side of the driver's door. But, that picture doesn't look anything like a data/VIN plate. I don't know what that is. Maybe somebody with more knowledge than I can chime in with the correct answer.
Sill plates usually refer to house construction, as in window sills.