Without the plate/cover, it would be difficult to drive the sight on, and guarantee that the blocker spring is vertical. If the spring is bent over (hopefully not bent) it may cause undue side pressure on the blocker, which could stick or extend with sufficient pressure. Only one way to know is install the sight and test the blocker movement by pushing in in and out, at a 45 degree angle, from the front. If the blocker moves freely, you should be good to go. Call S&W about the plate/cover and spring. they may send you one of each, or charge a nominal fee???
Bob