My one out of two Colt model AR6721 marked AR-15A3 Tactical Carbine law enforcement series, my older one shown was made in 2002 but I bought it new old stock in 2005.
Here's a best grouping pic of member Molon which I saved with his 6721 with free floating handguard and precision hand loads, he found out that it was the best production barrel made by Colt.
The 6721 marked as AR-15A3 Tactical Carbine had been in the Colt carbine model lineup since 1994 they were first designated as model R6721 for pre ban with "BD" serial number prefix and then during the ban changed the model number to AR6721 with "LBD" serial number prefix and as a carbine model in the law enforcement series.
There were only 134 made that was pre ban, and during the ban they also added the law enforcement restricted markings at the right side of the magwell.
If you download the 2016 Colt catalog in pdf format from Colt's site, they are shown on the catalog but not shown on the site.
My first 6721 carbine was made in 2002 and have large 0.170" dia. fire control group pins, while my newer same model made in 2010 have the small size mil-spec 0.154" pins.
The hammer and trigger pins was changed with most AR models to the small size in mid 2009 and are the same with the current production.
In 2005 up to the current latest production the bolt carrier group was changed to the M16 full auto carriers.
A different buttstock Colt-Rogers SuperStoc with locking levers was used sometime around 2012-2013 but went back to the M4 mil-spec stocks.