The Sap Cap isn't just a logo-less baseball cap. It's also a blackjack, one of those weighted leather weapons private eyes used to get socked with all the time back in the 1940s.
The bill is meant to be held like a handle, and the crown of the cap is heavily weighted, in case you want to bust some skulls but don't have any pocket space for a normal blackjack. It's not really clear what's actually providing the weight; apparently it's a "unique material that is 110% the density of lead and will not rust," which is thoughtful, because a rusty baseball cap is a dead giveaway that something weird is going on. [Product Page via Fashionably Geek]