Dating an SP1 upper?

[garrettbragg12]

I got bit by the retro bug and while building an M16A1, I picked up a SP1 stripped upper. I looked at a few sites but I'm still confused on whether or not it's an early or late upper. It makes a huge difference on the parts used on the build! Thank you!
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[Lee Enfield]

[QUOTE=garrettbragg12;353820]I got bit by the retro bug and while building an M16A1, I picked up a SP1 stripped upper. I looked at a few sites but I'm still confused on whether or not it's an early or late upper. It makes a huge difference on the parts used on the build! Thank you!
QUOTE]
It appears to be a fairly early Colt upper. Later uppers had letter codes in the right side webbing under the rear sight.
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It looks like you have faint Colt inspection markings, similar to this:
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Is that an SP1 upper, or an early AR15/XM16 upper?

If the front pivot hole is .250" it's AR15/M16, if it's .316" is SP1.