Well, without digging any of mine out, I can tell you there is a small difference in some of the rear blades. Some blades have a tiny "bump" protruding at the bottom. It allows you to align the blade in the rear assy slot. Look at your rear sight assy, and look for an extra little cut-out. If it has the cut out, your original blade had the bump. If not, your blade was "smooth." Of you get a rear blade that has the bump & your rear assy doesn't have the slot, then file off the small bump & use it.
I hope I got this clear enough for you.
Don't forget, there is a spring inside the rear hole for the blade to tension against.