The Mk. II was produced only between 1982-1987-ish.
Notice the grey parkerized finish: (The grips are not original, which are supposed to be the black plastic ones with the thumbrests)
The rib on the slide between the front and rear sights is unique to the Mk. II:
The hole in slide below the barrel was designed to help drain the Mk. II of water in case it was immersed during combat: