This is the kaweco fountain pen converter for their Sport model. Since that pen is considered a pocket pen, it uses this special smaller sized international converter. It functions by attaching to the nib and then pulling up on the back plunger draws ink into the pen like a syringe. You then leave the plunger extended and screw the nib section back onto the pen. Make sure there in not a cartridge inside the body, or it will not fit.