This is a 5mm allen key designed to fit in narrow spaces. I needed one for adjusting the rear brake pad of an iZip Dash electric bike; the clearance between the adjustment screw and the motor is too small for any normal tool. This little thing gets the job done without detaching anything. It's 4mm thick at the key end and the overall length is 70mm.
Note: this material is steel powder infused with bronze which is not nearly as strong as tool steel. I would not recommend using this tool for anything requiring significant force.