Premium and unique chest? Never heard that those exist on real worlds, however the update 1.33 is still fresh so it's possible those are a part of a secret quest or something.
If you're talking about tw-db site extra items, then:
1. Some extra items there drop from steel boxes (counterfeit gold weapons)
2. Some extra items exist only on beta (rocket turtle)
3. Some extra items are NPC only and/or can be given to a player by CM during an event or players can get them by using a redeem code when it's released (golden tomahawk)
4. Some extra items exist in the game but it's not possible to get them by any means yet (some pistol with a year number)
5. Some items in other parts of that site are in wrong category as are actually extra and currently can't be obtained nor is known how to get them (Rudolph)
In brackets I've given an example for each group. IMO tw-db should categorize those items so you can know if something you can get or not. Or completely remove them from the site.