The 33rd Mass probably was not engaged where the monument stands as it fought on the evening of July 2. The regiment was moved to the right after that engagement (to probably near the monument). Originally Col. Underwood wanted the 33rd's monument to be in a different place, presumably to the north along Wainwright Avenue, but that ground was privately owned. It is probably for this reason that there exists a large gap between the monument for the 33rd and the next monument to the north. Perhaps also why some of those monuments to the north seem too close together as they also could not be placed on private ground. There is a map from the 1880's or 90's which depicts this private lot. It can be inferred that the 33rd's monument should really be there or at least that is where Col. Underwood wanted the monument to be placed.
It is my belief that the next regiment to the left (north) of the 33rd Massachusetts monument when the Confederate advance began was the 17th Connecticut, which has its own monument further north (on ground it may have occupied on July 3). Also, when the 41st New York was driven back off the skirmish line, it formed on either side of the 17th Connecticut, which was in two lines behind the walls on either side of the lane. Reaching that conclusion involves an extended explanation, but there are two main points. First, the 17th Connecticut claims that it never budged from its position, and if that is true, the 7th and 8th Louisiana would not have been able to reach Wiedrich's guns so readily, if at all. Second, to reach the position indicated by their monument from their original spot facing north, the 17th would have had to move only a hundred yards or so to the right, but a few primary sources indicate that the regiment moved much further to the right, which makes sense if they were sent to defend against an approach from the north end of Culp's Hill - as was the 33rd Massachusetts. In addition, it appears there was only one wall along the portion of the lane where the 17th's monument now stands. There are other bolstering arguments. For instance, the crowd of stragglers who joined the 17th Connecticut can be none other than the retiring skirmishers of the 41st New York, but we can fix the left of the 41st New York with precision, since one of Battery L/1st New York's guns rolled down the hill on them, and that is further south along the lane. It also fits quite well with the fact that the 21st North Carolina did not reach the lane (in this scenario they would have been stopped by the 17th Connecticut), and it fits well with the known movements of the 6th North Carolina, which did break through and forced the left wing of the 41st New York to fall back to a wall running perpendicular to the one they occupied. All of the puzzle pieces fall neatly into place in this scenario, including the subsequent arrival of Carroll's brigade, which led to further realignment of the units.
But to accept the premise that the 17th Connecticut actually occupied the spot where their monument stands, on the night of July 2, introduces a number of not easily explainable circumstances, as I have pointed out. Plus, we would have to conclude the fragmented 41st New York made a heroic stand to stop the strong 21st North Carolina, which they never claimed. In fact, Carroll's brigade made that claim, but looking closely at the accounts and their casualty figures suggests they mainly collected the dead and wounded of the 21st North Carolina in front of the lane while mopping up, although they did help hurry the 6th North Carolina along on their retreat.
As for the 33rd Massachusetts, I conclude from Col. Underwood's official report that the regiment's right was not far from where their monument now stands when they fell back during the attack on the evening of July 2, although later that night after the fighting was over they shifted to the right so that their center would be near where the monument stands and their right wing would be placed along that fence and wall visible in the current photograph - perpendicular to their left wing, and quite close up against Stevens' left.