I agree: Scubapro Mk 5. Preceded, and very similar to, the Mk 10. Is it possible for the piston knife edge to cut completely through the HP seat? If that happened, wouldn't it sound like a "pop" at the moment it happened? And wouldn't bubbles immediately begin leaking out of the small hole?
Some HP seats were flat (no dimple) on both sides. Some people would flip the HP seat to the reverse side, as an expedient, when the piston knife edge had scored the seat too much. If the seat were scored deeply on both sides, would this increase the likelihood that the knife edge would cut clean through?
rx7diver