[jordanb]

Most aircraft are not built on assembly lines. They're moved to a station where a lot of sub assembly work is done and then moved to a different station. All the assembly work is done by hand. You can see that these planes are all at various stages of construction but some have their elevators and others don't. My guess is you're looking at the area where all the control surfaces are being installed. Probably all elements of flight control are installed here including the wiring harnesses, hydraulics etc.

The only aircraft I know of that was assembled on a moving "line" was the 777. It was a major innovation with that aircraft but not duplicated. Still it wasn't an assembly line in the sense of an automotive assembly line. Rather the aircraft were rolled along through the building very slowly as they were assembled.