The quickest way to get this is to unlock Leon's desk in the West Office. After you've used the combat knife to cut the tape on the box to the west wing of the station and raised the gate, head through the corridors and into the Operations room. Move through this room to climb up through the broken window.

Once you drop down, turn to the right and move forward, the door to the West Office will be in front of you. Move around the row of desks in front of you (be careful of zombies) to see a desk with two locks and a note. Enter the combinations for the locks and open the desk to get the weapon mod. Combine the mod with your handgun to get the trophy.

Left lock combination: NED
Right lock combination: MRG



Worth 10 Gamerscore.