I thought, though frequently wrong, that an A2 had the high front sight that was part of the gas block. Usually, but not always a carrying handle. (An A1 being the same but was the original design without forward assist or deflector.)
An A4 has a flat top. See below. Per Pegleg, yours isn't a single smooth rail, but has slightly two different heights. The rail on the fore end, or maybe the gas block, being slightly different from the rail on the upper receiver. This isn't unusal as the Ruger M77 bolt rifles have the same problem. And it's a pain to get scope mounts for them to work exactly right.
My latest AR has a full length float tube. The small gas block having no rail and fitting inside the float tube. A single monorail runs the whole length of the receiver and fore end.
So many configuration options in an AR, both a plus and a minus.