If you haven't seen it already,
here is how they work. Find some wire and bend an upstand on one end which is about the same height as the keyway, and add a loop of some sort for a handle. If the wire is too thick you can file a flat along either side until it enters the keyway. All you need to do is insert it all the way in, apply light turning pressure and move back slightly until you feel the latch. A bit more pressure against it will release the shackle. If you can't feel the latch, flip it around and try the opposite side. The cheap modern ones only lock from one side so it'll work one way or the other.
Your wire should look like the blue area I've marked on your
photo. If you file away the area of the key marked in red, this will do the same job. Keys are handy but they only work on one profile, so you're better off using wires or grinding flat steel (such as hacksaw blade) to shape. Have fun!
