About this detail of the Tiger
At the front of the turret there was a large opening for the gun mantlet. This rectangular opening was formed by the two crossbars of the turret front armour above and below, interlocked to the turret walls, and two cheek pieces welded at the sides.
This picture shows the right side of the opening seen from inside a late-model turret. I don't know the colour of the sides but my best guess is that it was sprayed with the base colour of the turret. At the outside bottom edge of the opening is the mantlet splash guard. The late-model turret was introduced just before the submersion system was deleted from the design. Therefore in all early-model turrets, this opening had to be made waterproof. Due to its size, it had a complex arrangement of fixed and movable sealing frames.
This picture shows an early turret. All around the opening, the armour has been shaved down by exactly 3mm forming a continuous flat surface. This surface was not exposed in the finished vehicle and was painted in red primer.
To achieve this flat surface, the designers had to alter the shape of the cheek pieces at the sides, and also cut a notch in the trunnions that supported the gun. Here's one of the trunnions for an early-model turret.
[2] Factory drawing 2AKF31861U1B11: Turmgehause (overall turret armour)
[3] Factory drawing 2AKF31861U1B12: Turmgehause (turret front plates)