 Complete the following phrase:  The first Zeros pre-series of 15 A6M2 went into operation with the 12th Rengo Kōkūtai in July 1940. On 13 September 1940, the Zeros scored their first air-to-air victories when 13 A6M2s led by Lieutet Saburo Shindo, escorting 27 G3M Nells medium-heavy bombers on a 