Hi Dick, The three position switch was used into the early '50's if I remember correctly. As you guessed it was so someone could move/use the car without access to the glove compartment or trunk, since one key fits all locks.
The '48-'49 switch has a tab under the key slot. If the tab and key slot is straight up and down then it is in the start/run position. If the top of the slot is to the driver and the tab is to the passenger that is the off/lock position. If the top of the slot is to the passenger and the tab it to the driver side that is the off/unlock position. In the off/unlock position all you have to do to start the motor is turn the switch vertical, without a key and press the starter pedal.
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