Right, the countersink is only needed on the FHC/DHC which had steel panel floors, to get the screw heads to sit down flush. The OTS with plywood floors would have countersunk holes in the plywood for the same reason, and probably did not have the countersink in the bars until the introduction of the FHC in '51.
As to packing pieces, we discussed that a few years ago. Should be in the archives.