This mod increases the weapon's damage by 25, from 80 to 105.
The vending code for this mod is found next to the Gala Event control panel in the bell tower during the quest Trigger the Gala Event.
After receiving the code, the mod can be purchased from any Sierra Madre vending machine.
The mod box is labeled "Gun Runners", yet it is stated that Elijah created the holorifle himself, not the Gun Runners, which assumes they couldn't have made the mods for it. However, this is because the holorifle focus optics doesn't have its own GECK image so gets the generic Gun Runners mod box image in its place.