I have a spare door latch mechanism and it has a bow tie slot for the key cylinder shaft, so I guess the flat part is correct. My key cylinder shaft does not have a flat part on the end, but it did work before the key cylinder broke. One side of the square socket broke off of the cylinder where the shaft engages, I don't think I'll be able to fix that. I'll locate a used key cylinder and have it keyed. I don't know if the shaft had a flat part on the end when new and it broke off, or not since it worked, that is still a mystery to me. I removed the door mechanism to compare it to my spare and it too has the bow tie slot for the key cylinder shaft. I can see the broken spring that lets my handle loose. One of these mechanisms will get a good cleaning and lube before re-installation.
