This is a simple tool handle for tool bits with a 3mm shaft. An M3 nut and 10mm button head screw is required for the assembly, and the tool bit of your choice. Just slide the M3 nut into the slot and push it into position, insert the tool bit and ensure the flat aligns with the nut, insert the screw, and tighten.

Note that the handle is not a steel part so do not over tighten the screw.

I have used the Hudy bits.

Hudy Metric Allen Wrench Replacement Tip (1.5mm x 120mm) [HUD111541] - AMain Hobbies