In the above description it refers to “the substructure comprises seven (7) 10 ga. H.R.B.A. formed steel cross sills and two (2) 10 ga. H.R.B.A.longitudinal sills, the cross sills being laid on their flanges while the longitudinal sills are inverted. Hardwood fillers are provided for the longitudinal sills.”

I’m not familiar with what H.R.B.A stands for and can anyone explain how the longitudinal sills would look with the timber fillers?

I think that HRBA = Hot Rolled Blue Annealed - but will be happy to be corrected.
The longitudinal sills are C shaped, wider than tall with the open face downward. The timber filling sits inside the sill (or in other words between the sill and the frame) to provide a bit of compliance/cushioning.