The basic patented design allowed a series of features to be deployed on the spar-frame that would later allow quick development of new models. The original Vespa featured a rear pillion seat for a passenger, or optionally a storage compartment. The original front protection "shield" was a flat piece of aero metal; later, this developed into a twin skin to allow additional storage behind the front shield, similar to the glove compartment in a car. The fuel cap was located underneath the (hinged) seat, which saved the cost of an additional lock on the fuel cap or need for additional metal work on the smooth skin.