Actually Highpower, what you have there in the first picture appears to be actually an early Colt SP1. Nice rifle BTW!! However, the military never used the 2-screw system and that was only used on the civilian models to prevent military parts from being used. Stupid I know, but that's a pretty good sign that it's a real Colt rifle.
Really though, unless you are looking at the pre-production prototype ARs, the first series to be adopted was the 601 that had the green painted brown furniture, a 1:14" twist barrel, duckbill flash suppressor, chrome slick-side bolt carrier, slab-side lower, and smaller delta-shaped charging handle. This was the rifle adopted by the USAF to arm the Security Police, replacing the M2 carbine in May of 1962.
The 602 series was the next upgrade by replacing the delta charging handle with the now current T-shaped charging handle, the furniture was changed from the green painted brown furniture to black and the rifling was now changed to a faster 1:12" twist as well as making the barrel slightly larger diameter (about .010") in front of the front sight base than the earlier 601 series. Another major change was the replacement of the thin duck bill flash suppressor with the heavier 3-prong style that served until the adoption of the bird cage flash suppressors.
The Army and Marines still refused to adopt the AR-15 as they lacked a way to manually force the bolt forward which lead to the development of the 603 series, first the XM-16E1 and later the M-16A1. Both featured the now familiar forward assist and the 1:12" twist barrels. Through the lifetime of the 603 series, the barrel changed to have chrome lines chambers then eventually fully chromed chambers and barrels. The XM still retained the 3-prong suppressor and also featured a chrome plated bolt and carrier as well as a buttstock with no provisions for a cleaning kit. A chnge to the lower receiver was the addition of the single ridge to prevent accidental magazine release to a point but mainly was now there to retain the pivot pin as a captive pin rather than the removable pin of the earlier 601 and 602 series. The later M-16A1 seen a change from the 3-prong suppressor to the birdcage, a phosphate finished bolt and carrier, a buttstock that now featured a compartment for a cleaning kit and a protective ridge around the magazine release button that was purpose designed to prevent accidentally releasing the magazine. This was the rifle that served up until the adoption of the M-16A2.
During this time, the Air Force parted ways, mainly due to the forward assist. They felt it wasn't needed and insisted that this feature not be included in Air Force M-16 contracts. This is the primary difference between the M-16 and the XM-16E1/M-16A1 series rifles. Production features otherwise mirrored the 603 series with the addition of the single detent pin tube, 3-prong suppressor, solid stock and chrome plated bolts and carriers with the later magazine protecting ridge, birdcage suppressor and phosphate coated bolts and carriers. One change that was incorporated with the 603 versions that wasn't carried over even in the later 604 variants was the Air Force decided to stay with the solid buttstocks rather than go with a butt trap feature.
Many of these old Air Force rifles from the 601, 602 and 604 series were finally upgraded to M-16A2 standards in early 2000s probably around 03-04 with the addition of a new A2 upper, fire control group, buttstock, pistol grip...well, pretty much most everything other than the lower, selector lever, buffer and buffer spring and a few odds and ends then were either stamped or electropenciled with "BURST" and the "AUTO" stamp was "XXXX" out.
I can remember we had 2 pre-604 lower A2 rifles in our vault that would occasionally come out into the classroom. I would wait for them to push the pivot pin out and when it fell out, go on an epic rant about how they broke the weapon and they would be charged with destruction of government property and get a chuckle out of their reactions as I then went on to tell them that no, actually these old lowers are still out there and if you take a good sniff, you can probably still smell dead VC on them so if they do get issued one when they deploy, just be aware of that pivot pin that will come out and to not loose it.
Really, there are 2 excellent sources for information on the early pre-A2 rifles. First is the retro section at AR-15.com. Lot of good information there! A second source to look into is R. Blake Steven's book, The Black Rifle. This book while expensive is a "must have" for anyone serious about the early years of the development and goes into a LOT of detail.
My 601 clone. All original parts except for the fire control group, lower Nodak Spud 601 lower receiver and the later style buffer. Edgewood buffers are EXPENSIVE especially for a part you never really see.
The early bolt catch on my 601.
The early Delta style charging handle. Yeah, easy to see why this got changed in the later series.
The green painted brown furniture.
The finish on the Nodak Spud lower is a little lighter than the original upper but it's not uncommon to see this especially in Air Force issued rifles.
Yeah, whatever works and what we have handy seemed to have been the motto even back in the day!
