Some older K-mount lenses have a "shroud" that protects the aperture lever that's larger than what tends to fit on a new camera. Look for a black protrusion sticking out of the back of the lens that's parallel to the range of motion of the aperture lever.
Other than protecting the lever, that shroud doesn't have any function as far as I know, and you may be able to remove it. Look for some small screws holding it on -- hopefully, screws that don't also hold anything else!
I have a lens that had this problem, and I removed the shroud with no further issues. Some people have chosen to cut the shroud down to size instead of removing it. Yikes! I wouldn't recommend that.
EDIT: dwb is apparently one of those bold metal-cutting mavericks!