Complex sentence: Between June 1978 and February 1979,15 Z2-class trams were built as an extension of the Z1-class order, they have few difference from the Z1-class built earlier.
Split into simpler sentences: Between June 1978 and February 1979,15 Z2-class trams were built as an extension of the Z1-class order. These have few differences from the Z1-class built earlier.