I believe what you may be viewing is wood added between the center sills to provide additional nailing area for the fixed center floor. The wood may have been added as a quick repair when the original nailers that sit on the main sills and between the steel ribs deteriorated. I do not think the wood would have been needed for structural support on any of the drop bottom gons that have the wood sills reinforced with car length pieces of C-channel steel. I am not sure how many cars were not reinforced with steel when the D&RGW rebuilt the cars in the mid 20's.