I am not sure when they started. From what I understand it is to mimic some sort fo US Govt. acceptance stamp or something? It is on the front of the magwell on my armalite rifle.
This is from the Armalite web site
WHAT IS THE STAMPED EAGLE ON THE FRONT OF YOUR RIFLES?
It is a factory inspection stamp similar to, and sometimes confused with, the government acceptance stamp. It doesn't include the additional characters of the government stamp.
It was initially used to mark Eagle Arms rifles that were built in the factory, to differentiate them from rifles built from Eagle Arms receivers. In addition, the company president didn't think that our rifles looked right without the stamp.
Because ArmaLite rifles are only available from the factory, the stamp denotes factory inspection, and to mimic government rifles in detail. The only ArmaLite's that intentionally omit the mark are those delivered to the government. This is intended to prevent confusion as to the source of a mark on the government rifles.