If anyone is interested in the developmental history pre-M16A1 and some of the more obscure variants that have been out there along the way,
Retro Black Rifles is a pretty cool website, with photo references and information on what parts you'd need if you were going to try to build a replica.
There's also a forum about retro rifles on AR-15.com which sometimes has some good information. (Including a guy who built a repro of an M231 Firing Port Weapon.)