A twin 4.5-inch (113 mm) Mark 6 gun mount was fitted forward. While the "Leander"-class was planned to be fitted with the Sea Cat surface-to-air missile, "Penelope" was completed with two Bofors 40 mm anti-aircraft guns as a temporary substitute until Sea Cat could be fitted. A Limbo anti-submarine mortar was fitted aft to provide a short-range anti-submarine capability, while a hangar and helicopter deck allowed a single Westland Wasp helicopter to be operated, for longer range anti-submarine and anti-surface operations.