What year is the car?
Starting with the 1969 model year, GM had a reverse "back-lock" linkage on manual transmission vehicles. This linkage connected the reverse lever on the transmission to the switch you have pictured. IIRC, it served a dual purpose:
1. turned on the back-up lights when the car was put into reverse
2. allowed the ignition switch to be rotated to the "Lock" position only when the transmission was in Reverse.
There was a bell-crank and rod linkage from the reverse lever on the transmission to the lever (below the floor) on the column to make the system work properly.
Many cars have had the system removed over the years.
1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base