I am not sure your question was answered, and I never looked at the IC flat specifically, but can speak to another GSC cast flat often referred to as a “Commonwealth” flat.
Keep in mind that castings varied by owner and year built.
There were two kinds of “holes” in the deck of the flat. The obvious ones were the square holes cast in as stake pockets. Some cars had four at each end; these have two.
The other, more numerous “holes”, were, from what I was told, “stress-relieving” depressions cast in to minimize chances of cracking in the slowest-cooling portion of the casting. There are obviously numerous circular depressions in the
surface of the casting inboard of the draft gear and bolster.
The flats I crawled over had depressions that did NOT go the entire depth of the casting. I could not figure out a way they were used to secure loads, but I do not know these cars. They collected a lot of dirt.
Some GSC cast flats did have tie-downs welded to the side sill at the rim, in the form of steel circles. See attached.
I cam across this photo of IC flats being assembled at the IC's Centralia car shops from frames bought from General Steel Castings.
I've never seen these holes in the steel at each end of the deck.
Maybe I've just missed them in other photos.
Jeff White Alma IL