Wolfskin1,
That is an old bug. It is listing the 4 .303s (none of which are in the tail), but not the 2 .50s.
The Lacaster MkIII in AH has:
Position 2: Nose Turret, 2 x .303 Machine Guns
Position 3: Dorsal Turret, 2 x .303 Machine Guns
Position 4: Tail Turret, 2 x .50 Machine Guns
So you see, the 4 .303s listed in the hangar are those that are on the nose and dorsal turrets, there is no mention of the guns on the tail turret.