That's a harder question to answer than you could imagine, even though the facts are simple.
Let's just say the "original" was an evolution that began with the first production model and continued into the late 1980s. It normally has a single sided safety lever and small M1911 military style sights (exceptions here). It had either an internal or external extractor depending upon its vintage. The Mark II had ambi safety levers, plastic "thumbrest" grips, a rib on the top of the slide, better sights, and a "drainhole" at the front of the slide. In both cases, the rear sight was dovetailed, but the front sight was staked on to the slide. The Mark III is the basic current production gun. It has the ambi safety, may or may not have the same grips as the MK II, has no rib on the slide and has an enlarged ejecton port.
The reality is that the Mark II and Mark III were/are simply continuations of the same evolution that has been ongoing since the first gun came out of the factory door.