Mid '70s is about the age of mine as well. My brother bought it new and shot the heck out of it. Still shoots fair but it is pretty loose. When I first saw the screw on the bottom of ScottMcM's frame I though that he had figured out a way to tighten his up.
With a thread title like this, it just was a matter of time! I'm surprised it stayed on course as long as it did!
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek