Complex sentence: The War Office authorised the construction of three prototype models in April 1941 and it was considered a successful design, with the Tank Board of the War Office ordering 1,000 to be constructed in September.
Split into simpler sentences: The War Office authorised the construction of three prototype models in April 1941. The new design was considered a success, and the Tank Board of the War Office ordered 1,000 to be constructed in September.