Our friends over at Looserounds.com have published a good article on the Colt SPII series which provides some good history and clarification.
The Colt SP-IIs ( R6400, R6401, R6500)
The Colt AR-15 Resource
"A site dedicated to the history of the Colt AR-15 rifles, carbines and associated items"
The Colt AR-15 Resource 