The final version of the AK-47 receiver is the Type 3 built as either the Type 3A (fixed stock) or the Type 3B (underfolder). Quick differences between the Type 2 and 3 receivers include the deletion of the buttstock socket, the addition of the sling attachment at the gas block, and the alteration of the back of the receiver profile. Here is a Type 3A:
And a Type 3B underfolder:
By 1959, the Russians had solved the problems with the stamped receivers and introduced the Type 4A receiver, which is stamped. This is the basic AKM receiver from which all AKMs and AK-74s originate. The Type 4A is made from a 1mm thick piece of stamped steel that is folded into the receiver shape and heat treated. Thicker, 1.5mm steel is used on Chinese, Yugoslavian, and some Bulgarian AKMs. Since it is a stamped piece of metal, all the parts that had been machined into the milled receivers now have to be made separately, installed, and then secured with rivets. The rivets all over the sides of the receiver are a simple way to spot a stamped receiver. Other changes made include the addition of a magazine guide dimple on each side of the receiver on 99% of AKMs, just above the mag well opening, 1000m sights (usually), recontoured barrel, new gas block, new (narrower) front sights, and a relocated bayonet lug. Although the original AKM receiver was used for the 7.62 round, it lent itself to other calibers, including the 5.45x39 (AK-74) and 5.56. Here is a typical stamped Type 4A receiver.
The two rivets on the right (and their counterparts on the opposite side) are used to secure the rear trunnion, which secures the buttstock to the receiver. The one rivet on the left is a reinforcing rivet that spans the width of the receiver.
The three rivets at the front of the rifle (and those on their opposite side) secure the front trunnion to the receiver. On the latest version of the Type 4 receiver, the Russians have added an additional rivet to the front trunnion. Just above the left-most rivet you can see the pin that secures the barrel (which is pressed into place) to the trunnion.