This is a fairly small 4 pin brass padlock. I oppened it once but am having trouble repeating the feat. The third pin from the front has by far the most tension and lifting it even a little bit and applying tension will produce a click. Looking at the key, however, it should be a fairly high pin. Additionally, the corresponding ridge on the pin is cut wierdly, with a "ledge" cut into the top of the pin like so: __/\
/ \
Is this a special pin?