copied from: http://www.ak-47.us/AK-47Receivers.php
Ak-47s have two basic types of receiver. The folded sheet of steel called a stamped receiver and the solid steel block drilled and shaped into a milled receiver.
The first AK-47s were stamped receivers. They were strong and easy to build. But in an attempt to improve their strength the Russians turned to a milled receiver for many years. The labor and material required to make a milled receiver were too much and the stamped receiver was again adopted. There is some debate over which is stronger or better, and both have advantages and weaknesses.
Milled Receivers are strong, will not twist, should last for many years of shooting. They also cost more, and are heavy. They have fewer stock options and few parts kits are available for the milled receivers.
The first picture of two sides of one receiver is stamped. The second two pictures are different versions of milled receivers.
Basicly, dimples and rivets = stamped, machined depressions = milled