Are you sure it's not M42 now? From memory Sears used M42 and then K mount. If it doesn't fit the K mount then my first guess is M42. U could stand for Universal mount.
Unless you're attached to the Sears lens I wouldn't spend money on a M42 mount. Very good M42 mount 135mm lenses tend to be fairly cheap.
When you hold it on the camera does it make infinity focus?