Your sight is fine. It is a war time expedient designed to speed up production. The sights are interchangeable. The Milled adjustable sight is an easy upgrade and fairly easy to find. I have two No.4's. One has the same sight as yours the other has the milled adjustable. I found another adjustable on E-Bay they seam to run any where from $35 to over $100 depending on the markings. You can also try Numrich Gun Parts or Springfield Sporters. At Numrich look under SMLE.