You must call Colt with the serial number. You can also find a copy of the Blue Book of Gun Values and look it up since you did not provide the serial number.
You will have to call COLT
No such sn in the AR-15 series
You need to call Colt
you will have to call Colt to find out.
You will have to call Colt
1974
1971
About 1976
Proofhouse.com has Colt sn data.
1986
1970
In 1974.