If this goes on much longer, B had better build the next few batches with uninstalled stab gear and a few panels off - the better to install possible mods for certification.
Modern assembly line processes are designed to build a product in a specific sequence and there may be problems progressing if a particular stage is missed. Permanent fittings such as rivets may be needed after the missed stage which would require extensive work if it had to be redone. Some parts such as control surfaces could easily be left off and fitted at a later date if required but if major disassembly and reassembly is needed it may be better to stop the line.
Just in time inventory control is used in the automotive industry, and occasionally critical parts need to be delivered in a chartered aircraft to keep the line going. A few thousand dollars to fly in a couple of crate loads of ignition sensors is nothing compared to the thousands of dollars a minute halting production would cost.