Your's is probably an A2. There are not a ton of differences between them all. Along when several little changes and changed within models.
A1: A1 sights, which are pretty rudimentary, but they worked. The A1 sights were contained within the carry handle. Can be id'ed by a large wheel on the right side of the carry handle. Little bit shorter butt stock. Different pistol grip.
A2- Newer sight system windage and elevation adjustable. What you find most common today if not a flat top. Different pistol grip, added "fingergrooves". Slightly longer buttstock.
A3- Removeable carry handle aka flat top upper.
In the early years, they had slabside lowers. Which meant they did not have a mag fence and didn't have material in some areas. Also, slickside uppers which were minus the forward assist. They added the forward assist, which was tear drop shaped when the A1 was still top dog and added the mag fence during this time also.
I'm fairly certain the brass deflector didn't come around until the A2, but I could be and probably are wrong on that one. The charging handles were more of a delta shape on some models, versus the straight design now. Another big one is the flash hider. They had the three prong, then the A1 birdcage, then the A2 birdcage. The A1 had slots all the way around, the A2 has a solid bottom. From what I understand this was done to prevent dust from being kicked up from the muzzle blast when shooting prone. The three prong was just that, resembled some kinda demented hot dog cooking utensil.
Find a good book on the Ar15 or M16 and read away, there's a lot of history on these weapons. Makes for good reading.