Valve gear for the water supply pipes to Bangor, I think. There used to be a cast plaque on the railing, Bangor Water Company or similar, if memory serves.
The supply, taken lower down, at Ceunant, below Gwernydd, was the source of the Cholera outbreaks that affected the Bangor area. It was a family living in Gwernydd that were the carriers and their effluent was entering the river just down stream of Cae Swings but above the extraction point. The settlement pools used to be extant, opposite Nant-y-Greaen.This was also the where the isolation hospital was! The solution was to move the extraction point up to the point you have identified. Later developments established the treatment works at Ty Slatters. A popular local sport used to be smashing the supply pipes on the Gwern, below Gerlan, and creating enormous fountains!
I've put one of my views of the subject in My Personal Album.