Slip hook, I believe is the term for that sort of release.
Mine is waaay smaller than yours
![Roll Eyes ::)](http://www.primitivearcher.com/smf/Smileys/default/rolleyes.gif)
I made a miniature siege engine way back, had pictures with some pet gerbils 'manning' it... well 'Gerbilling' it I s'pose
![laugh :laugh:](http://www.primitivearcher.com/smf/Smileys/default/laugh.gif)
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