It is added to the player's Pip-Boy inventory after speaking to Sgt. Contreras at the Camp McCarran supply shack about increasing his weapon supply (level must be 12 or higher), and passing a Speech/Barter check of 50 or pay 50 caps. Downloading the weapons manifest from the NCR supply database terminal upstairs also gives the disk (no level required).
I need to increase the number of weapons and crates of ammunition from our previous arrangement. Things might heat up with Caesar's Legion again and we'd like to be prepared. We need to increase the number of weapons by 25% and ammunition by 50%.