the m1931 was in production in 1935-1939. it was gradually improved until 1937, so late production pieces differ from the early production ones. in 1939, the gun was replaced in production by an improved variant, m1931/37, which combined barrel of the m1931 with the carriage of the 152 mm howitzer-gun m1937 (ml-20). the exact number of pieces manufactured is not known; estimations are 450-500 pieces.