Complex sentence: It was not till the mid- 1920s that production restarted with 330 M1918s being produced and M1918 became the heaviest mobile artillery piece in the US Army until its replacement, the 240 mm howitzer M1, was introduced in 1943.
Split into simpler sentences: It was not until the mid- 1920s that production restarted with 330 M1918s being produced. The M1918 became the heaviest mobile artillery piece in the U.S. Army until its replacement, the 240 mm howitzer M1, was introduced in 1943.