This is sort of a companion piece to my earlier post on upper receivers.
What we have here are the major variants that Colt issued to the US Military.
This is the R607 stock. It's Colt's first attempt at making a stock that can be shortened. The design of the AR recoil system prevents the use of a more traditional folding stock. It was heavy, awkward to change position and expensive. It did not last long. (Picture taken from this thread in the Arfcom retro sticky on carbines)
This is the first major production collapsible carbine stock. The receiver extension that is still in production was introduced with this model (although the number of positions has changed). It is aluminum covered in vinyl acetate. The first weapon it appeared on was the XM177E2 "submachine gun". This is actually the second version of this design, the original was merely anodized the same color as the receivers; that appeared on the XM177 and XM177E1. The vinyl coated one was in production from the late sixties to the early eighties.
The aluminum proved to be a bit expensive and could break if dropped in the butt. So Colt introduced the "fiberlite" stock. This stock was made from the late eighties into the early 2000's. The early versions of the M4 and M4A1 carried this stock.
The modern M4 stock! It has a bit more butt area and a sling swivel to match a standard rifle.
Now to rifles.
This is an M16 stock as would be found on early R603 and R604 models. This is called a "type D" stock by us retro AR types because there are three earlier variations for the R601 and R602 rifles. All Air Force M16 (R604) use this stock and it was standard on the XM16E1 and M16A1 until around 1971.
The Type E or M16A1 stock. The sling swivel no longer swivels and a trap-door was added to the butt for storing a cleaning kit. It's a bit heavier than the Type D too.
The M16A2 stock. It's similar in design to the Type E, but is made from different materials and is about 5/8" longer.
Of course we must mention the aftermarket. This is a MagPul CTR.