Genuine M-B lockable fuel tank caps must have been an option back in the day before the outer doors were tied in with the central locking system. I've got one on the '74, one I found on a U-Pull W115 a while back. When locked the cap just spins. On W114 and W115 access to the fuel cap is not part of the vacuum central locking system - It's just a door that flips open with no provision for locking.
