Does that switch have a small handle that hangs down below the key slot? If so, the 12:00 position is the "ignition on" position. Turn the cylinder to the left and remove the key, and you've got "off and locked". Re-insert the key, turn the cylinder fully clockwise and remove the key. That's "off and unlocked". You can use the tab to turn the cylinder to the middle position and start the vehicle without a key. Some commercial trucks, school buses, and such had that optional switch setup so the vehicle could be operated without the need for a key. The 1958 Chevy school bus that I rode for 3 years in middle Tennessee back in the early 1960's had exactly the same switch.
Jerry
