All of the ones with GI stamps have good non-bending plates and measure .030
Promags have cheesy floorplate, measure 0.30 and have 6 spotwelds on the lug, but I cant find a stamp..
When I had these rifles I don't recall having problems with them.
The one next to the 10 rder in my pic with a question mark on the paper has a good floor plate but measures .032. and the lug only has 4 spot welds. Fake??? don't know. It has no markings that I can find. The 10 rounder has a good plate and measures 0.30 with 2 spotwelds on the lug but I can find any stamping on it. It too, looks like someone experimented on it.
Count the spot welds on the lug on the mags that you have.
I was always told that if it's not 5 or 6 it's not 'real'
Then I found a dozen blued non hardened gunshow junk mags that have the lug stamped out of the body. I think I got ripped off on those..
My computer wouldn't let me download Different's guide but here is another link
Pictorial M14 Magazine Identification Guide Version 1.8 by Lee Emerson (eBook) - Lulu