I have always been curious but never enough to ask this.
I have a Colt pre-ban HBAR purchased new by me in '86.
The receiver is marked AR-15 A2 HBAR Sporter cal. .223 Ser. SP21XXXX.
Now, I read here and elsewhere, where four digit model numbers are used to describe such rifles. Owners/shooter rattle these numbers off like their DOB's.
Could someone please tell me the correct model # designation for my rifle?

Thanks...

Probably an SP2.  See: www.biggerhammer.net for info.

www.biggerhammer.net/ar15/variants/

Thanks guys. That was very fast in deed!
It looks like it's an R6600 and that's kind of what I originally thought from what I had read.

Thanks again...that's a great reference site BTW.