An apparatus for permitting inspection of container processing includes a one-piece elastic peripheral lip seal that seals a transition between a window and a mounting frame. In its un-deformed state, the lip-seal, which has first and second strip-shaped lip-sections, has an at least partially sickle-shaped cross-section in its un-deformed state. Both lip sections extend outward toward respective first and second edges, the second edge being a sealing edge. A thickness of the second lip-section decreases towards the sealing edge. The lip sections curve around an axis oriented along a longitudinal direction of the lip seal. The first lip-section defines a concave curve along a first surface of the lip seal, and the second lip-section defines a convex curve along the first surface.
