Your cannon's limber will either have a set of Pole Chains or a set of Pole Straps at the end of the limber pole.  The Pole Straps are a serious piece of equipment meant to hold the wheel horses to the limber and help serve as the brakes for the vehicle.  Made by Austin Fox, these include huge custom forged buckles and are made of three layers of heavy harness leather.  Each order is for one pair of Pole Straps, enough for one limber.