I’ve been looking at adding an SP1 Carbine to the collection. Currently one has my attention but I have a little concern with the serial number possibly be to low.

Bottom line.... What was the first year the carbines showed up and any idea what the possible lowest serial number a person could find in an original SP1 Carbine?

http://retroblackrifle.com

Some say September 1977 was first year. I'm thinking more like 1978 time frame. SP83400 was the starting serial number for the 1978 year production. With that being said there wasn't anything in the serial number that designated  if it was a carbine or rifle.

https://www.ar15.com/forums/ar-15/-/123-304266/?

First carbine posted in the SP1 database was 1978

Ok thanks. The one I’m looking at is 86,xxx so looks like it’s good to go. Plan to inspect it this weekend.

Thanks Guys....

Nice, my earliest is 88k and just picked up a 87k.  Post pics when you get it.

Can Colt confirm if it left factory as a Rifle or Carbine?
Sure would had to drop the coin on a made Carbine.

I don't have a answer for you if Colt can tell you if it left the factory as a carbine or a rifle. Maybe someone else can chime in and answer that.
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This pictures were taken of my 1979 SP1 serial number 1025XX
I would check the staking of the castle nut end plate area to see if that all lines up. Also check for the dab of paint. This is what the factory did.
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Second thing I would look for is the barrel marking. This will be found underside of barrel toward the A1 birdcage flash hider. C MP B. I believe around 1980 production SP1 carbines the writing was spelled out and was on top of the barrel.