I’m not sure if I got this for completing a Side OP or if I got it form interrogating a guy in Da Wialo Kallai. The OP takes place in Da Wialo Kallai so land near there. Head into the camp and find a lone soldier to interrogate and he should point out where the blueprint is, it will be a check mark on your map. Head there and grab the box, it is a little white box near some books.