The car had then as it does now most of it's frost floor. The frost floor(it has other names) is simple tongue and groove boards nailed to the bottom of the sills. Sometimes insulation was placed in between.
What you see in his drawing at the rear is the sill extensions. These bolt to the body sills and hang below the frost floor and extend out past the end of the body to support the rear platform.
The car has 6 sills like any car of the time frame. 2 center sills, 2 intermediate sills, and 2 side sills.
I am the curator of equipment and rolling stock at the Colorado Railroad Museum we are currently restoring this car. What dimensions do you need? I can help.