This is a release latch for 2 litre water rocket bottles. The latch is designed to slide over a standard 21mm (21.3mm/21.5mm/ 27/32") external diameter waste pipe used as a combined fill rod/launch rod and slides comfortably inside the neck of the water bottles. No seal is required for most 2L bottles but it's useful to have a seal to make launching easier.
The hinged parts are attached to the hinge plate with M2 x 12mm screws and you should slide a 21mm x 1mm O-Ring over the pipe and sit it in the recess of the hinge plate.
To install the latch grab some 32mm PVC waste pipe couplers from your local DIY shop. Their internal diameter should be 36mm. Slide them over the launch rod first, then glue (superglue/CA works) the hinge plate on the launch rod using a bottle to get it lined up.
Once the glue is dry install the O-ring, screw on the hinge parts lining up the threads to make sure you get them in the right order. Slide the 32mm coupler up the tube and it should squeeze the three threads together ready to receive the bottle.
Screw the bottle onto the threads, pressurize, cover your ears and pull the coupler down.
I'll upload some photos shortly.