Complex sentence: The most common variant was a twin turret design fitted with two machine guns, however a number of other variants were developed including a vehicle armed with a 3 inch gun and an anti-aircraft variant armed with a 13 pounder AA gun.
Split into simpler sentences: The most common variant was a twin turret design fitted with two machine guns. However, a number of other variants were developed including a vehicle armed with a 3-inch gun and an anti-aircraft variant armed with a 13 pound AA gun.