Look in the upper right corner of the fret, just below the product name. It is labeled Open Bridge Details. That goes on the top of the bridgehouse, the square you have highlighted yellow, just forward of the stack.
The bridge wings stick out on either side of the bridgehouse, allowing full side to side access & visibility. The parts on the lower left of the fret go on the kit parts you noted (B11/B12). If you look closely at the arrow on Loren's plans it is bent sharply, pointing down. He means the deck flooring,