The 8×4 wings, parts 3933 and 3934, come in two main variations. In the original design, the slope of the wing edge cut off part of the area where a stud might fit. When people inevitably tried to attach a stud at that point, it would damage the pieces.
LEGO redesigned the part with a small notch inside the wing edge to accommodate a stud. According to Bricklink inventories, the new version first appeared in the early 1990s. These are designated 3933 and 3944.
Unfortunately, some Bricklink sellers do not take care to distinguish which version they are listing for sale. This is easily seen by the number of listings for the old version in colors that were only produced well after the new version was introduced. If you are completing or restoring a Classic Space set, confirm with the seller that they are offering the correct version.