I followed the link, seems they don't know much either. It IS marked "M15", and serial number (wont be posted anywhere) is apparently hand stamped and pretty crudely I might add since the numbers are there with different depths and the baseline is uneven.
I admit this is my first and only aloominum gun, I have one handgun with a polymer "stock" but I am DEFINITELY a walnut/steel guy for the rest of the collection with the single exception of my Norinco SKS. The AR's fit/finish is vastly superior to that Chinese unit, but the materials just seem cheaper in comparison (for the receivers, the barrel is SS and the handguard is beautiful, a 7-inch floating tube).
For example the finish (anodizing?) has been marred on top of the A2 carry handle, and the biggest mark was made by the handguard when the rear pin was pulled and they came into contact. There are a number of small areas around the dust sheild (?) too where metal-to-metal contact has resulted in exposing the silver. So maybe it is a poor finish rather than cheap aluminum? I am used to a lot of aluminum-billet parts for my cars and this stuff just seems really soft is all.
Thanks for the feedback.