The two-gun Light Defender was an early version of the three-gun "Heavy Defender", seen in early screenshots about a year before the game's release. There was also a "Light Interceptor," but this eventually became what we now know as the Scout. The Light Defender, having little utility, was dropped from the game. 

We can imagine that the Mk.1 Light Defender was a very early fighter developed on Kharak, but retired before the Mothership launch because it was made obsolete by the development of the Scout, which outclassed it in every category. Later, however, the need for an anti-fighter craft and development of improved targeting systems and addition of a third gun brought the Shield back into service as the Mk.3, a venerable but cheap and reliable defense fighter. 

The Shield series of defense fighters is one the sturdiest front line designs employed in the Kushan fleet. Drives and weapon systems have evolved over time, but the basic hull design of the original Shield strike craft has yet to be surpassed in terms of economy and durability. A new state-of-the-art targeting system and an upgrade to the gimbal-mounted turret systems have been incorporated into the current Mark 3b version of this classic strike craft, and should serve to keep the Shield effective in combat for the foreseeable future.