This is much more likely to be a camera-body light leak, not the lens. On an SLR, the lens is on the other side of the shutter from the film, so stray light from the front only reaches the film during the brief exposure. A true light leak usually comes from the camera back or seals, where light can seep in continuously.

Also remember the image on the negative is reversed/upside down relative to the camera body. A leak showing at the top-right of the photo points to the lower-left area of the camera back. On a Pentax K1000, that means checking the back-door seals, especially the lower seals and the hinge-side seal.

A good test is to put a small flashlight inside the camera, close it, and inspect the outside in a completely dark room after your eyes adjust. If light can leak in, it can leak out.

Most likely fix: replace the K1000’s light seals. It’s a common maintenance job and often best to replace all the door seals rather than only one section.