Slapjacks, also known as a "sap" or "Blackjack", are flattened and weighted leather baton-like devices. The head is filled with a lead slug and lead shoots, and the body has a piece of spring steel and lead shoots - combined, the device is stiff but also has a bit of flex, which helps it make a nice slapping thud when it hits a table or similarly hard surface. You'll be surprised at just how much of a wallop they pack. It's an old and somewhat nostalgic design. The sap is a great self-defense weapon one level before deploying a firearm.
The SAP I create are beautiful durable and very effective.