The Aero Commander 680 was developed from the Aero Commander 500. 

Model 680FL Introduced in May of 1963 as the “Grand Commander”. This was the first Twin Commander model featuring the lengthened fuselage seen on later Turbine and JetProp powered models. Two sections were added to the plane, a 44 inch section forward of the front spar and a 30 inch section aft of the front spar. This added over six feet to the length of the plane, bringing overall length to over 42 feet. The Twin Commander offered leading cabin area space per occupant and the plane’s gross weight rating increased to 8,500 lbs.

Model 680V  Introduced the large under wing picture windows that offered rear cabin occupants an unprecedented outward view from the plane. These windows would become a Twin Commander trademark in future years.

Model 680T  The growing use of jet engines throughout the 1960’s by commercial airlines led to demand by the general aviation market for increased aircraft speed. This market demand increased in the late 1960’s with the introduction of small jet engines for use in jet powered business aircraft. In response to this demand for increased speed, the Company undertook design and certification work for the first turbine powered Twin Commanders during 1964 and 1965. The first turbine powered Twin Commander was the 680T called the “Turbo Commander” that used Garrett AiResearch TPE331 turbine engines. The increased power of the Garrett AiResearch engines allowed for a gross weight rating of just under 9,000 lbs., better flight performance characteristics and increased take off power. The high gross weight rating and additional power required certain configuration changes to the airframe that altered the flight characteristics of the plane. The 690A and 690B models that followed the 680T were the primary turboprop Twin Commander models.

Source: Wikipedia