AR15.com can give you the date of manufacture of your rifle.
The Sporter II was a civilian AR15 manufactured by Colt. I remember they started marking the Colt AR15 "Sporter" when some states and the federal gomt started the uproar over "assault rifles." This was even prior to the weapons ban of 1994.
Obviously, the Sporter II is more recent in manufacture than the "Sporter" which is sometime after 1985 when the original Colt SP1 was discontinued. Usually the Colt civilian rifles are a mix of old and new features as compared to military standard.
For example, my SP1 made before 1985 has the M16A1 flash hider, the M16 upper receiver, sans forward assist, and the triangular handguards used on both M16 and M16A1, so it cannot be exactly compared to any official military model. The bolt is also notched for the forward assist, even though none is fitted.
The round handguards are a feature of the M16A2, although they will fit on any M16 model. The shorty AR15 with collapsable stock has always had the round handguards, however.
I believe the "A3" is not an official designation, but I may be wrong. It's been a long time since I researched AR15s.