Complex sentence: The aircraft were built in 1930-1931, and the designation changed to PZL Ł.2 (Ł for "łącznikowy"-liaison), or Ł.2a.
Split into simpler sentences: The aircraft were built in 1930-1931. The designation changed then to PZL Ł.2 (Ł for "łącznikowy"-liaison), or Ł.2a.