Is that a model 512 on a NEA handguard? Looks the same as this one I've got here that does not fit. The one with the full base. Did you grind down the stops on that handguard? Was thinking of doing the same but wasn't sure if it would compromise the structural integrity of the aluminium.
Hey, I'm not sure of the model, but it has the flat base (no clearance). It did not fit the NEA rail, so I filed the end of the rail into a square to accommodate the sight. It holds zero perfectly without any play. My advice would be to be conservative in your filing. Better to have to incrementally remove more material than to take too much off.
Good luck!