LBD00XXX
Looks to be a model 6721
should be a 16" barrel 1/9 twist heavy barrel, A3 flat top such as here on this link
http://www.impactguns.com/store/COLT-AR6721BF.html
for more Colt serial number information
http://www.sr25.com/ar15/ar15serial.html

nice carbine
Thanks. It is an A3 with "Colt C MP HBAR" stamped on the barrel.
From the URL above...
[FONT=Verdana, Arial, Helvetica, sans-serif]
Serial Number - BD[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]
Prefix [/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]
Model Name - AR15A3 Tactical Carbine[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]
Sample Model-[/FONT] [FONT=Verdana, Arial, Helvetica, sans-serif]6721[/FONT]
What does the L represent?