Might be just a special code sheet included with the collection. Unique stuff is like that sometimes, and may not be available on the store at all.
Here's an old example:
Medal of Honor (2010) Limited Edition PS3.
This game has MoH Frontline on disc (only platform that had that I think).
Also had early BF3 beta access, some extra maps, early access to upgraded in-game weapons, and online pass all activated by one code afaik. LE also had at least one (maybe 2 I can't remember now) exclusive weapon not obtainable any other way: H&K MP7.