Complex sentence: On 1 September 1939,219 vehicles had been delivered including prototypes, 71 behind schedule but production increases soon allowed Panhard to reduce the backlog, at least for the hulls.
Split into simpler sentences: On 1 September 1939,219 vehicles had been delivered including prototypes, 71 behind schedule. However, production increases soon allowed Panhard to reduce the backlog--at least for the hulls.