I have an opportunity to buy a 3-digit ser. # SP1. (4XX) I would rate it 95-97%, good finish, 3-prong, slab-side, etc..
The only red flag is there are NO markings on the barrel. None. I will continue to research this but do you guys have info or advice on early SP1's? TIA.

Does the ser no read like this - SP 004XX ? That would be correct for a first issue SP1.

Does it have the features normally present on a first issue SP1?

Take a close look at the barrel. It should probably be stamped with the number 12 on the underside just about an inch back from the FH. It should also have a small letter M and the triangular Colt proofmark stamped on it's right side between the FSB taper pins. It would not have a chrome bore and chamber.

HTH

Thanks for the reply. It has all the features of an early SP1. The ser. # is 004XX.  I will inspect it again with better lighting and magnification but I'm 99% sure there are no marks. My initial inspection showed no markings on the BBL. I think the barrel has been changed. Too bad because it is a nice piece plus NO play in upper/lower! Thanks again.

I can't help but be curious, what sort of price are they asking?  I'm not good enough on early SP-1s to give you an answer on the barrel, sorry.

He's asking $1000.00. Which I thought was reasonable given the condition and low ser. #. But if it's been re-barrelled...

Pull the handguards and take a look at the teeth on the barrel nut. A rebarrelled piece will very likely show some little marks on the teeth.

Re the barrel markings that you didn't see, my experience w/early Colt ARs is that production markings are often very faint, and take a good strong light to spot.

Don't give up on it yet