This old cover of SWAT caught my eye the other day. The gun on the cover is what Colt refers to as the R6400 as their factory model number. The keen eye will notice A2 furniture but an A1 rear sight. That’s because it uses the upper and lower of the older SP-1s but was upgraded with A2 fore arms, grip, buttstock, delta ring and the new A2 government profile barrel. Also a A2 flas hider and a front sight. The barrel of course uses the 1/7 twist t use the then new “SS109”, AKA, M855 green tip. Very new at the time. Cover date shows 1984 so this was even before the Army adopted the M16A2 rifle.
The roll mark on the lower was AR15A2 SP-II. As the older uppers ran out, Colt then sold the R6401 which was the same gun but with the actual A1 upper with a forward assist. Next came the R6500 which was same gun with the C7 upper. That was the A1 upper with forward assist but with a case deflector as used by the Canadian rifle.
Finally Colt would use up all those uppers and sell the R6550 which uses the true A2 upper to go with the rest of the upgrades. The lowers would continue to be the older slick side SP1 no fence A1 profiled lowers for a while before being changed over to the current A2 lowers.
During this time, Colt stilled used the “large hole” upper and lowers for the front pivot pin. Contrary to internet experts this wasn’t done because colt hates you but was something the federal gov ( read ATF) made them do so as to be able to sell semi auto Ar15s to the public. This was foisted on them all the from the start of the SP1 civilian sales. Other changes were the semi auto carrier and a few other things. But that’s an article for another time.
There are offset pins to let you mate a large hole upper or lower to a modern or “small hole” or “milspec” upper and lower and it works pretty nifty even though you will need a screw drive to separate the upper and lower. Eventually the gov relaxed these crazy requirements and the ever law suit adverse Colt stopped all modifications to civilian guns and now sells the with standard pin holes, trigger groups and with M16 BCGs. Just as God intended.