It has been documented that Colt had four bolt carrier designs in the evolution of the development of their piston driven operating system which culminated in the LE6940P carbine. Chris Bartocci of Small Arms Solutions has a good article on Colt's development of their piston system here:
The Evolution Of The Piston Operated M-16/M-4 RifleIn his article, he has this photo of the four different Colt piston bolt carriers.
Attached FileI was fortunate to have the opportunity to buy one of the 3rd design piston bolt carriers. In these photos you will see the 3rd design piston bolt carrier displayed with the 4th design bolt carrier that is in my LE6940P. Be advised that a low number of 3rd design piston bolt carriers were used in early LE6940P production before 4th design piston bolt carrier use was implemented.
Attached File Attached FileThe most stark difference between the two designs is the presence of the solid design carrier key of the 3rd design that was not staked. This was the part that absorbed the rearward motion of the piston. From what I understand, the carrier keys would shear off which resulted in the 4th design being implemented into production.
Attached File Attached FileA couple of photos will follow in the next post.