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Old November 30, 2019, 13:21   #51
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Join Date: Dec 2008
Location: UTAH
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Originally Posted by 2barearms View Post
So can someone explain the 3 screw vs 4 screw uppers?
4-bolt = ER (Early Release) 1st gen, manufactured circa June '07-ish. Will have some/all of the following Swiss parts in addition to US parts: imported cold hammer forged barrel (Supposedly. Personally, I have never been able to confirm this, as there are no roll marks. And, no factual documentation verifying this info.) Swiss gas block w/roll mark, Swiss gas piston, Swiss numbered (but not matching numbers) bolt carrier group, Swiss fire control group w/proof, Swiss lower parts kit, and finally a Swiss pistol grip...

I believe somewhere around JS 5XXX-6XXX the imported Swiss parts started to dry up. So, they might have some of the above listed import parts, but tapper-off after this. I cannot confirm what serial number they switched to 3-bolt uppers. I've not ever owned or handled a 556 in this range. No clue if they still had a few leftover pistons, or FGS to put into these or not.

If anyone knows exactly when they switched to 3-bolt uppers, I would love to know.
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