The lower end of the seatback is connected to the seatfloor via a piano hinge. Three hinges are in fact installed to allow the seatback to be reclined into three positions.
The installation needs some time for all the measurments and drillings to be done.
Those with good eyes may notice some lines and holes that should not be there below the hinges on the part on the right. It's due to an initial error in taking reference for the first holes. The nice thing is those holes are interfering with nothing, they will make the left side lighter and will be concealed under the seat cushion.