A wide recess in the external circumference of the titanium ring design, is where a steel band that is a part of the external TITAN Cyclops 2 support structure, is wrapped around - 2 such bands, 1 front, and 1 rear, attaching the TITAN Cyclops 2 hull assembly to the support structure:
The titanium front end "bell" cap "hatch" part and the rear end "bell" cap part are bolted to their respective mating flanges of the aforementioned titanium rings. At 1:05 / 14:18 in video 'Missing Sub' . . ., you can see a port side view of an OceanGate Cyclops [1 or 2, I am uncertain], with the front "hatch" opened, and Stockton Rush sitting at the opening.
Looking at the opened "hatch" flange surface area, there appears to be a ring of some type, concentric with the flange circumference, that is probably a seal. No seal is apparent in the corresponding forward-facing flange surface of the titanium ring. The "hatch" flange and its mate, the titanium ring flange, are hinged together at the 9 o'clock position.
David Pogue, at 2:55 / 6:16 in video 'Safety is relative' says that there are 18 bolts that secure that "hatch," but only 17 bolts are used - the top-most bolt position at 12 o'clock, is apparently not used ("way up high, and they say there is really no mathematical difference"). [The bolt heads are 6-point, not 12-point in the images and videos that I have seen.]
The front end "bell" or cap part is unique in design - in contrast to the rear end "bell" or cap, in that, the front "bell" includes the viewing portal. And, during manufacturing of the titanium front end "bell" cap, some stress and potential for fatigue is introduced in forming the neck that you see in the image, that presents the base for attaching the viewing port assembly.
IMHO, the front "hatch" hinge, plus the missing 18th bolt at 12 o'clock, plus the stressed-neck of the "bell" adjacent to the viewing port, will all affect how the deep water pressure works to distort the "bell" shape of the front hatch. Distortion will affect the ability of the plane of the mated flanges to remain perpendicular to the central axis of the hull. That disruption can affect the integrity of the titanium ring and Carbon Fibre hull cylinder assembly.