In Fallout: New Vegas, codes for items from DLCs are written with xx at the start of the item code, standing in for a unique identifier known as a DLC code.
A DLC code is a two-digit number that tells the game which DLC an item comes from. It varies from player to player because it's determined by both the number and the load order of the DLCs you have installed.
Why do some item codes begin with xx?
Item codes that begin with xx refer to items from DLCs. The xx is a placeholder for the DLC code, which is unique to each player.
This uniqueness comes from the DLC load order, which affects these numbers. The only "fixed" IDs are those from the base game, which always start with 00 because it's always the first to load. You can see the base-game codes on the item codes list.
Console method
Open the console and click on an object native to the world (one that came from a DLC). The first two numbers of that item's code are your DLC code. New to the console? See how to open the command console.
Brute-force method
If that doesn't work, try each of the following in place of the xx field until an item appears:
01020304050607,08,09… and so on.
One of these numbers should be the correct DLC code.