AR Upper Receiver Basics?

[LRRPF52]

In the original guns, there are at least 4 different types of uppers before we even get to the A2 uppers:

[SIZE="3"]Armalite Prototypes with "trigger" charge handle:[/SIZE]



[SIZE="3"]Colt Model 601, the first production AR15:[/SIZE]


[SIZE="3"]Colt Model 603 upper used on the XM16E1 and M16A1:[/SIZE]


[SIZE="3"]Colt 723 and Colt Canada C7 upper:[/SIZE]


All photos referenced from http://retroblackrifle.com/