I recently acquired a Colt 1911 Mark IV lower series 80 all complete from a friend that is brand new. He also had a slide that is brand new it says on the slide it's a Mark IV series 70 gov. I thought it was for the most part ready to go minus the internals until I flipped it over and it looks like a series 80 with plunger hole. Can someone advise why it says series 70 and appears to be an 80?