That's a warded lock. It doesn't have any pins inside of it like a typical pin tumbler lock does. You'll need to use a warded lock pick, which is basically a skeleton key by definition.
A lot of pick sets from Southord come with a set of warded lock picks like these:
http://www.lockpicks.com/warded-padlockpicks.aspxThey are the same kind I have, and they work quite nicely.