Slip hook, I believe is the term for that sort of release.
Mine is waaay smaller than yours
I made a miniature siege engine way back, had pictures with some pet gerbils 'manning' it... well 'Gerbilling' it I s'pose
Took some stop action pics in a darkened room with a flash linked to the arm of the engine... before digital cameras.
It shows the action of the sling lobbing a marble.
Del