Tested on a White-listed Private Server by 7 Players that tested the Loot.
Missing Items in DayZ 0.55:
-M4A1 (except you re-install your server for new Heli spawns but that would be abusing)
-M4 Magazines (same as above)
-Blaze Double Rifle
-AK Rail Handguard
Bugged Items in the Loot Table.
worth it looking for it even though it is a Key Feature of the Game_
-7.62x51 (Mosin) Ammunition (Found in Total 3x20 Rounds in Total on the Server / No Respawn in 7 Days) White you find enough Ammunition for not existing Weapons (Winchester/Longhorn)
-9mm Magazines for MP5 Every Police Station is full of them.
-Police Cars spawn Items next to the Car
-Varios Buildings don't spawn Items
-Respawn of Items not existing everywhere untlill not respawning at all