This is a toy knife that uses gravity to extend and retract.

You can pivot the lever to allow the blade to slide up and down the handle.

The lever holds the blade extended or retracted when closed.

Other hardware:

1 m3 nut and m3x10 bolt used to attach the bushing to the knife

2 m2.5x6 bolts to attach the lever to the handle

The blade should be able to slide in and out of the handle freely, might need to sand it a bit.