Beat all 75 missions. Then, Press Start to pause game play, then press Start + Select to resume the game in a first-person cockpit view.
In the mission Hour of 16:00, the part called "Protect Allied Squadron", look in the right passage, on top of the first cliff. It's hard to see from the ground, you may have to look around a bit. You'll find a left arm/laser sword part called 'Ywl03lb-tardos'.
Thanks to Revolution readers Sekeja and BigZapper!