Complex sentence: These aircraft were the largest operated by the Imperial Japanese Army Air Service and their existence within it was kept secret and they were issued their out of sequence Kitai number when they were finally revealed in 1940.
Split into simpler sentences: These aircraft were the largest operated by the Imperial Japanese Army Air Service and their existence within it was kept secret. As a result, they were issued their out-of-sequence Kitai number only when they were finally revealed in 1940.