Check this thread out (will have to scroll up to read from post #1) There is a lot of information in a number of threads started around that time frame First AR 556/223 - #18 by Craig6
Also look at www.pewpewtactical.com do a search for AR-15 upper and lower assembly.
Edit: https://www.pewpewtactical.com/build-ar-15-lower-receiver/
https://www.pewpewtactical.com/build-ar-15-upper-receiver/