Don’t doubt yourself too much. I am not an expert or even a collector of Colt AR-15’s. I qualify myself as a poor student of the model. Your description just didn’t fit what I normally find. The more you look at Colt’s the more you become convinced there is no norm. To cloud the issue even more I’ll tell you of another SP1 similar to yours. Some years ago I was attending the Allentown, Pennsylvania, Forks of the Delaware guns show. At this show was dealer who was offering for sale a number of Colt AR-15 variants all chambered for the .222 Rem. cartridge. While I had seen Colt’s in this caliber before, all had been of the same model, similar to the R-6500, he had several and each was a different variant. I inquired as to were he had found them. His story, for which he had documentation, was that a friend of his was a financial backer of Colt. The friend had made a large loan to the cash strapped company. In return he was allowed to go to the factory and purchase what firearms he wanted. The dealer then point out to me a Colt SP1 sitting near the .222 rifles. The rifle had the appearance of a late 1970’s early 80’s SP1. The rifle look mint except for a large gash in the buttstock . The dealer explained that the rifle had been used by a Colt Vice President as a door stop, at the factory. I examined the rifle and was surprised when I noted the serial number SP00232. This would indicate 1964 production. Here in is the problem, the rifle was assembled from parts that were of much latter production than 1964. It had the bird cage suppressor, chrome lined bore, A1 style buttstock, notched bolt carrier, all fire control parts were of much later manufacture, all this and the finish of the parts matched . It also had documentation showing it had just come from the factory. So I for one will never tell anyone that their Colt is positively not in the configuration it left the factory in. I try to look for patterns in the production, but as you see they don’t always hold true.